Redness of the skin when seeing the sun may be caused by physiological factors, solar dermatitis, systemic lupus erythematosus, etc., which should be analyzed according to the symptoms. 1. Physiological factors: the infrared rays of sunlight have a thermal effect, and prolonged irradiation on the skin will cause capillary expansion, resulting in skin congestion and redness, which is a normal physiological phenomenon. 2. Solar dermatitis: after sunlight ultraviolet irradiation, proteins and nucleic acids in the cells will absorb a large number of ultraviolet rays, resulting in complex photochemical reactions, causing necrosis of white skin cells. This causes inflammatory reactions of the skin, such as redness, swelling, pain and swelling. 3. Systemic lupus erythematosus: patients with systemic lupus erythematosus are sensitive to light, and ultraviolet irradiation of the sunlight on the face or other skin tissues may induce or aggravate skin redness. There may be other reasons for the redness of the skin when seeing the sun, which should be promptly consulted by a professional doctor for a clear diagnosis and treatment.