In order to protect your skin and correctly choose the right cosmetics, you must first understand the nature of your skin. Usually, skin can be divided into three types, namely oily, neutral and dry, but from a medical cosmetic point of view, skin can be divided into six types according to its water content, pH value, sebaceous gland secretion status, etc. I. Normal skin: It belongs to the ideal skin. The sebum and water content of the stratum corneum are appropriate. The water content of the stratum corneum is more than 20%, the pH value is 4.5-6.5, the skin is smooth and delicate, shiny, fine pores, moderate oil and water, looks rosy, smooth, no defects and elastic. Less sensitive to external stimuli, should not be wrinkled, makeup is not easy to fall off. Mostly seen in adolescent girls. Dry skin: mainly the natural moisturizing factor in the keratin forming cells and sebum secretion is reduced, the water content of the stratum corneum is less than 10%, the pH value is greater than 6.5, the skin texture is fine, thin, pores are not obvious, sebum secretion is less and even, no greasy feeling. The skin is relatively dry and looks clean, delicate and beautiful. This kind of skin is not easy to have acne, and strong adhesion, makeup is not easy to fall off after makeup. However, dry skin cannot withstand external stimuli, such as wind and sun, and the skin flushes and even burns after being stimulated. Easily aging wrinkles, especially in front of the eyes, the corners of the mouth is most likely to have wrinkles. Oily skin: Due to excessive sebum secretion by sebaceous glands and imbalance of sebum and water content in the stratum corneum, the water content in the stratum corneum is less than 20% and the pH value is less than 4.5, the appearance of the skin appears thick and pores are obvious, and some pores are large and resemble orange peel. Sebum secretion, especially in the face and T-shaped area can be seen shiny; skin texture rough, easy to be polluted; antibacterial power is weak, easy to have acne; poor adhesion, easy to fall off after makeup; more able to withstand external stimuli, not suitable for aging, facial wrinkles appear later. Fourth, mixed skin: the presence of two different nature of the skin at the same time for mixed skin. Generally in the forehead, nose, chin for oily, large pores, oil secretion, and even acne can occur, while other parts such as the cheeks, showing the characteristics of dry or neutral skin. V. Sensitive skin: delicate and fair skin, less sebum secretion, drier. The distinctive feature is that contact with cosmetics can easily cause skin allergy, redness, swelling, itchiness and so on. Allergy to hot sun, pollen, mosquito bites and high-protein foods are also easy to cause. The reaction to the patch test for sensitive substances is positive, mostly seen in allergic people. Sixth, problem skin: the so-called “problem skin” refers to the acquired, due to adverse environmental stimuli, inappropriate treatment, care, poor nutrition, imbalance and other reasons caused by a variety of facial skin disfiguring lesions. The common problem skin are: eczema, hormone dependent dermatitis, rosacea, acne, seborrheic dermatitis, chloasma, freckles and other skin diseases that affect beauty in life, collectively known as problem skin.