By adjusting the wavelength, energy and pulse width of the laser, the laser penetrates the surface of the skin and reaches the root of the hair follicle, the light energy is absorbed and converted into heat energy that destroys the hair follicle tissue, thus causing the hair to lose its regeneration ability without damaging the surrounding tissues, and the pain is mild. It can be seen that laser hair removal has become the safest, fastest and longest lasting hair removal technology. Because of the three phases of the hair growth cycle, only the hair in the anagen phase can be effectively removed by the laser, but not the dormant hair follicles. Because the hair follicle is in the anagen phase, the hair matricellular cells divide rapidly and the most melanin is present during this phase, which is extremely sensitive to the laser. The hair growth cycle varies from one area to another, so there are significant differences in treatment. In addition, the number of treatments depends on the hair density, color, follicle diameter and the percentage of non-anagen hair follicles in the treated area. The density and growth of hair varies with gender, age, individual and site. Men generally have denser hair and grow faster than women. Hair growth is fastest at the age of one year, and as age increases, hair density decreases and growth rate slows down, probably due to endocrine regulation. In addition, the density of hair in each part of the same body is different, such as the density of hair follicles on the forehead and cheek is double that of the trunk and extremities. At the same time, the hair growth cycle is different in each area. These are all factors that affect the effectiveness of laser hair removal. The best results were obtained in the axillary area, followed by the bikini area and the extremities. Some of the patients with poor hair removal results were due to the inability to adhere to the schedule or to the full course of treatment. Take the collar for example, most of the patients are male, because of the tight working time, they can’t follow the required treatment, often after the treatment, the hair starts to thin out, and delay or stop the treatment. In fact, the hair follicle is not completely destroyed, and the hair is regenerated after a month or more. In addition, because the hair growth cycle is different in each area, patients must follow up regularly. If patients do not follow the doctor’s instructions, they will miss the best treatment time for the hair growth period, which will lead to poor results. Secondly, the patients with poor results are mostly darker in color, which may be due to the fact that the specific wavelength of light emitted by the instrument is not only absorbed by the melanin in the hair follicle, but also by the melanin particles in the skin. Therefore, it is difficult to control the energy of the treatment for such patients, and the pigment particles disperse the energy, and the hair follicle damage is not ideal. Again, the different characteristics of different parts of the hair make the treatment different. The treatment effect of coarse black, too dense hair is not ideal, probably because the light wave has not reached the hair follicle, but has been absorbed by the dense, coarse black hair shaft on the surface of the skin, the distribution of dense hair follicles, absorbing light quickly after heating up, which can cause skin damage. In addition, if the hair is too fine, light and sparse, the laser energy is not absorbed well and the effect is not good. The better hair removal effect in the extremities and armpits is related to the shallow hair follicles and the degree of hair follicle cell destruction after laser irradiation. The upper lip was less effective because of the finer and lighter color of the hair, less melanin in the follicle, and poorer absorption of laser energy. Lastly, the age factor, which accounts for the total number of patients under 35 years of age, is one of the main reasons for the poor outcome of patients in this age group, as they have a faster rate of hair regrowth.