The common causes of darkening of the skin in a circle around the neck include acanthosis nigricans, skin melanosis, and hyperpigmentation. 1. Acanthosis nigricans: the cause of this disease is still unclear, may be related to genetic factors, drug factors, disease factors, obesity and so on. It occurs in skin folds, manifesting as deepening of skin color, light gray, light brown or dark brown, surface dryness, roughness, skin thickening and other symptoms. Most of the patients have a better prognosis after regular treatment, while some of the patients do not take active treatment, and their condition aggravates with a poorer prognosis. 2. Melanosis: the cause of this disease is not clear, mainly manifested as gray, black pigmentation on the skin, aggravated by sun exposure, occurring in the neck, can appear in the neck skin color black symptoms, after regular treatment, can effectively improve the symptoms. 3. Hyperpigmentation: this disease is mainly caused by heredity, endocrine disorders, metabolic abnormalities, inflammation and other factors, occurring in the neck, there may be a deepening of the skin color, can become black. It is recommended that patients according to their own situation, to the regular hospital dermatology consultation, a clear diagnosis, in accordance with the doctor’s instructions for reasonable treatment, to avoid delaying the condition.