The use of lasers in cosmetic dermatology is not limited to skin rejuvenation and the treatment of cosmetic skin disorders. How to achieve this goal more effectively and safely is a constant goal for plastic surgeons and a major challenge for the field. Skin rejuvenation is essentially the process of combating skin aging, while cosmetic skin disorders include skin pigmentation, vascular disease, trichomegaly, infectious skin diseases, and skin redundancies. From the point of view of tissue optics and skin aging mechanism, the action of laser is realized by heating the target tissue, which is called photothermal effect. The basic histological changes of skin aging are the reduction of dermal collagen fibers, elastic fibers and other organic components and interstitial tissues, resulting in skin atrophy, dryness, wrinkles, skin laxity and skin pigmentation changes. The dermal tissue is heated to produce new collagen directly or by activating a series of related cytokines that cause collagen cells to produce new collagen and remove pigmented skin surface damage by direct vaporization or altered metabolism. Laser technology undoubtedly has a unique advantage in skin beauty. 1, laser skin rejuvenation technology progress 1.1, laser dander technology In 1989, RP Abergel and LM David used C02 laser to irradiate the patient’s hand, the patient’s local skin quality improved significantly, since then, laser skin peeling or laser dander was carried out on a large scale abroad. In 1997, the author started to use the ultra-pulsed CO2 laser to perform skin resurfacing surgery. From the efficacy, the patient’s skin texture did improve significantly, as evidenced by a more lustrous and delicate skin texture, tightening, wrinkle reduction, and pore reduction. However, it was soon found that the probability of local complications of pigmentation in postoperative patients was high, which was related to the patient’s skin type. Pigmentation was more common in patients with type III or higher, especially with pigmentation problems such as melasma, and patients suffered more pain during surgery and had a longer recovery time after surgery. Except for the treatment of type III or lower and depressed acne, this technology has been basically abandoned in China. In order to reduce the adverse reactions after laser surgery, the Erbium-YAG laser was subsequently used in laser dermabrasion surgery, which has a longer wavelength (2940 nm) and better absorption by water, so penetration is shallower and postoperative complications have been reduced, but still does not solve the problems of intraoperative pain, postoperative pigmentation and missed work. 1.2, long pulse width laser technology Since heating the skin can increase the content of skin-forming components, we just need to choose not to destroy the epidermis, but directly heated dermis laser can achieve the purpose of “skin rejuvenation”. At present, there are a variety of lasers used for skin tightening and wrinkle removal cosmetic treatment, the purpose is to heat the dermis and stimulate its collagen cell activation to produce more collagen fibers, such as Nd:YAG laser (1064 nm, 1320 nm), Er:YAG laser (1540 nm), semiconductor laser (940 nm, 1450 nm), pulsed dye laser (585 nm, 595 nm ), etc. Their common features are: (i) longer wavelength and strong tissue penetration; (ii) longer pulse duration (pulse width), which makes the tissue slowly and continuously warm up; (iii) most of them are equipped with epidermal cooling devices to protect the epidermis from damage. The clinical application of this type of laser has been seen to be effective and has been supported by experiments by domestic scholars. However, it is not without regret that the cosmetic effect of this type of laser is not very certain, and more clinical and laboratory studies are needed. 1.3, image beam, fractional laser In order to overcome the shortcomings of peel-off laser technology causing great damage and non-peel-off laser technology penetrating shallowly and without obvious effect, Anderson proposed a new concept of fractional laser. The basic principle is to divide a continuous laser spot into a series of discontinuous, uniformly distributed focused dot arrays. The diameter of each dot is 70-120 μm, and the interval between the dots is much larger than the diameter of the dots. At these points, the energy density is high and the laser penetrates deep into the dermis through the epidermis, forming a series of columnar microscopic thermal zones (MTZs). In this zone, intracellular water absorbs the 1.54 μm erbium laser and warms up to necrosis. The surface stratum corneum, however, contains little water and remains largely transparent to light, preserving the barrier effect on the skin. The unaffected normal cellular tissue between the exfoliated areas acts as a “bridge” and they rapidly initiate the healing process. Partial skin remodeling and regeneration is achieved through the regeneration and migration of new keratin, collagen and fibroblasts, which replace the necrotic tissue. Each treatment area covers 13% to 20% of the total treatment area. After 4 or 5 treatments, all the skin is regenerated. In fact, the initial effect of the treatment becomes apparent 1 month after the start of the treatment, and the full effect of the treatment becomes apparent after 6 months. The improvement of the skin condition is very obvious. Since then, various forms of fractional lasers have been introduced. Different wavelengths of laser can achieve different treatment effects, such as spot removal, wrinkle reduction, skin tightening and scar repair. From the mode of action on the skin, they can be broadly classified into the following 3 categories. 1.3.1, exfoliative laser CO2 laser can fully vaporize the epidermis, and its heat can reach the dermis, and play the purpose of heating the collagen of the dermis. It is a good treatment for skin laxity, wrinkles and scarring. It can be effectively and safely used for the treatment of photoaging of facial skin. It can also improve all types of superficial scarring. Since this type of laser has significant exfoliation and deep penetration, adverse reactions still occur, including transient erythema, edema, flaking, decrusting, acne-like eruptions, and hyperpigmentation. And there is more than 1 week of missed work period. 1.3.2, micro-exfoliative lasers The representative laser is the erbium laser, which can vaporize the epidermis and play the role of superficial skin treatment; it can also heat the collagen of the dermis and play the effect of skin tightening. Although this type of laser vaporization and peeling is shallow, repeated treatment of the same area will increase the depth of vaporization, thus bringing about the above-mentioned adverse reactions. 1.3.3, non-ablative laser There are various lasers of this type with wavelengths of 1320-1600 nm, and their common feature is that they penetrate deeper and heat the dermis in fractional form, and the surface vaporization is not obvious, therefore, safer. It is used for skin tightening, wrinkle removal and superficial scar treatment. Larger energies have a vaporizing effect on the epidermis as well, which may cause blisters, hyperpigmentation and even scarring. Non-ablative laser skin tightening and wrinkle reduction technology mainly through the selective absorption effect of light and biostimulation, after acting on the treatment area, the laser energy is selectively absorbed by water or hemoglobin, causing the dermal collagen fibers to be heated to appear contracted and denatured, and inducing a wound healing response in the dermis, resulting in the orderly deposition of collagen, thus improving skin elasticity, reducing wrinkles and tightening the skin. Christiansen and Bjerring used low-density non-ablative fractional CO2: laser for rejuvenation, and ultrasound imaging showed a 72.7% increase in dermal density and an 80% improvement in perioral wrinkles, confirming that non-ablative fractional CO2: laser can significantly improve wrinkles, skin texture, and punctate pigmentation. 2.Laser treatment for melasma Laser treatment for melasma has always been controversial, and a variety of lasers have been used clinically, but there is still a lack of convincing evaluation of the treatment effect. In recent years, large spot, low energy Q-switched pulsed lasers and fractional lasers have been used to treat melasma at home and abroad. 2.1. Q-switched Nd:YAG laser units use Q-switched Nd:YAG laser with large spot and low energy scanning to treat the patient’s face in order to improve skin quality, remove superficial wrinkles and reduce pores, especially for melasma. The treatment method is divided into the following 2 forms. 2.1.1 “White porcelain doll” adopts large light spot (spot diameter 6~7 cm) and low energy (1.4~4.0 J/cm2) to carry out sweeping treatment on the face, each treatment can be seen to sweep the skin lesion more than once, 1 or 2 times a week, 5~10 times a course. It has good efficacy in treating melasma, and can achieve the effect of skin rejuvenation and wrinkle removal, whitening and spot removal. 2.1.2, “black face doll” is called soft peeling treatment by some foreigners. The method is to apply a layer of toner in the treatment area, and then use a large spot (spot diameter 6-7 cm), low energy (1.4~2.0 J/cm2) for sweeping treatment on the face, and finally use a smaller spot (diameter 4 cm), low energy (1.4~4.0 J/cm2) for sweeping treatment on the face. It can achieve the effect of whitening and wrinkle removal, skin tightening and pore reduction, and can even effectively treat inflammatory acne. 2.2. Fractional or beam laser In 2003, 1550 nm erbium glass fractional laser was applied in the treatment of melasma. Katz et al. treated eight patients with melasma with the erbium glass fractional laser and performed long-term follow-up, which showed that five cases were effective and three cases were recurrent. Niwa Massaki et al. achieved satisfactory results with a single high-density treatment with a 1927 nm fractional laser and concluded that the efficacy was superior to that of Fraxel. Jalaly et al. treated melasma with a low-energy fractional CO2 laser and the results were superior to those of a low-energy Q-switched 1064 nm laser. In summary, lasers have promising applications in facial skin aesthetics. The application of new lasers has enabled a good balance of treatment efficacy and safety. However, there are still many issues that need to be further discussed, such as the selection of indications, the setting of the treatment course, and the objective criteria for determining the efficacy, especially the selection of treatment modalities for yellow skin characteristics is still in the exploratory stage, and more detailed clinical and laboratory studies are needed.