Summer rash on the legs, may be insect bite dermatitis, polymorphic sun rash, eczema and other diseases. 1. Insect bite dermatitis: If the skin on the legs is bitten by mosquitoes, their saliva contains acidic substances, which can cause local inflammatory reaction after entering the body, appearing fusiform edema mass, accompanied by itching, so that the legs appear to have the phenomenon of red rash. 2. polymorphic solar rash: the disease is usually caused by allergy to ultraviolet light, strong sunlight irradiation of skin exposure areas, such as the face, upper limbs, chest, V area, legs, etc., causing an acute allergic reaction, the affected area can appear erythema, papules, itching and other symptoms. 3. Eczema: the cause of the disease is not known, is internal and external multi-factor interaction leading to superficial dermis and epidermal inflammatory diseases, can occur in any part of the body, so that the legs appear erythema, papules, papules, etc., leading to the phenomenon of red rash on the legs. It is recommended that patients with red rashes on the legs in summer go to regular hospitals in time to get a clear diagnosis and standardized treatment under the guidance of doctors to avoid delays.