Sudden darkening of the face may be caused by physiological factors such as sun exposure, and to a lesser extent may be related to diseases such as acanthosis nigricans and melioidosis. 1. Physiological factors: If the face is exposed to sunlight for a long time, sudden darkening may occur. 2. Acanthosis nigricans: the cause of this disease may be related to heredity, low immunity and obesity, etc. Clinical manifestations include deepening of skin color, dryness, roughness and itchiness, etc., and some patients may experience sudden darkening of the face. 3. Melanosis: its cause is more complex, may be exposed to photosensitive substances, constipation and other factors. Clinical manifestations of the damaged skin gray-purple, grid-like hyperpigmentation plaques, some patients will have a sudden darkening of the face. There are many reasons for the sudden darkening of the face, and after excluding physiological factors, patients are advised to seek medical examination to clarify the cause of the disease.