Contact dermatitis is the most common type of facial dermatitis, caused by skin allergy after contact with certain types of substances. Common allergens include metallic mercury, cosmetics, hair dyes, facial cleansers and so on. If the allergens are identified as soon as possible and care is taken to avoid re-exposure, the condition can be minimized, the treatment effect can be enhanced, and recurrence can be effectively prevented. The patch test is the most common test used to diagnose contact dermatitis, and this method has a high accuracy rate. The doctor places common allergen testing reagents in separate patch testers and applies multiple patch testers to the patient’s back or upper arm. The patient himself removes the patch after 48 hours and visits the doctor one day later (one day after the removal of the allergy symptoms are more obvious), so that the doctor can judge the skin condition. If the skin of the patch contact area appears red and edematous, it is positive, indicating that the patient is allergic to such substances, and these red and edematous symptoms are generally mild and can subside naturally; the patient needs to be checked again after the 7th day from the start of the patch test. This is because some patients may develop allergic symptoms later, i.e., delayed allergy. The commonly used patch test kits can screen for 20 common allergens, and the Department of Dermatology at Peking University People’s Hospital can perform primary screening tests for 60 allergens common to dermatitis, as well as cosmetic patch tests (containing 60 common allergenic cosmetic ingredients) to further clarify the cause of facial dermatitis. Some patients with dermatitis caused by exposure to certain types of substances followed by sunlight exposure can also undergo a light patch test if they are suspected of such photosensitivity. This test is a little more complicated, and patients need to apply two sets of dressings symmetrically on their backs, remove them for 48 hours, expose one side to long-wave UV radiation and the other group to no radiation, and then compare them to see if there is a difference. It should be noted that if the patient is in the acute phase of dermatitis, with obvious redness and oozing, it is not appropriate to do the patch test; and if the patient is undergoing anti-allergy treatment, such as oral anti-allergy drugs, hormones, etc., the test needs to be done after stopping the drugs, generally antihistamines need to be stopped for three days, and oral hormones should be stopped for two weeks. Patients with facial dermatitis should have a clear diagnosis as soon as possible to find the cause of the disease. When the cause is unknown, if you want to relieve the symptoms, you can use a moisturizing mask, or do facial cold spray; when it is clear that it is an allergy to a substance, or the dermatitis is more serious, you can use topical hormone creams under the guidance of a doctor, but not for too long, otherwise it will easily cause hormone dependent dermatitis.