Erythema multiforme is an acute inflammatory skin disorder with a polymorphic rash that may be associated with mucosal damage. The severe form has severe mucosal and visceral damage. The lesions are polymorphic, occurring in spring and fall, and are more common in adolescent females. Diagnosis needs to be made on the basis of its clinical symptoms. The edematous polymorphic rash and the site of occurrence are generally not difficult to diagnose, and it is easier to confirm the diagnosis if there is a typical iris-like rash. The following describes its clinical manifestations. 1, the rash is polymorphic, the beginning is mostly erythema, and later there can be wind, papules, nodules or blisters, purpura and so on. The size of erythema is like a lentil or a nail cap, the color is bright red, the center is dark red or purple red, and can be fused with each other. Some of the erythema center can subside, forming a ring, or appear overlapping blisters, shaped like iris, quite characteristic. 2, the lesions are mostly symmetrically distributed on the back of hands and feet, palm and plantar, wrist, elbow, knee, face, neck and buttocks. In severe cases, the whole body, can involve the mouth, eyes, external genitalia and other mucous membranes, mucous membrane redness, swelling, vesicles, ulcers and so on. 3.Self-awareness of itching or burning sensation, often accompanied by systemic symptoms of varying severity, such as fatigue, headache, fever, arthralgia, and so on. 4, the duration of the disease is 1-2 weeks in mild cases, and in severe cases, the disease can subside in 3-4 weeks. Seasonal relapses are common, with spring and fall being the most common. The disease is more common in adolescents, especially in females.