Neck skin blackening may be caused by physiological factors, such as long-term sun exposure, etc.; it may also be caused by pathological factors, such as acanthosis nigricans, Riel’s disease of melanosis, etc.. 1. Physiological factors: ultraviolet rays have a strong penetrating power, which can penetrate the cloud layer and irradiate up to the dermis, promoting the proliferation and differentiation of melanocytes, leading to the secretion of a large number of melanin particles and making the skin dark. 2. Pathologic factors: (1) Acanthosis nigricans: mainly characterized by excessive skin pigmentation and velvety thickening. The etiology is not yet clear, and it may be related to heredity, metabolic disorders, and massive application of drugs (such as nicotinic acid drugs, glucocorticoid hormone drugs, etc.), etc., which can lead to the phenomenon of darkening of the skin of the neck. (2) Rayle’s melanosis: the cause of this disease is not clear, may be related to a variety of factors, such as long-term use of photosensitive substances in cosmetics, dietary vitamin deficiencies and sun exposure, etc., histopathology suggests that the basal cell liquefaction degeneration. The disease may appear as darkening of the neck skin in the form of brown patches with indistinct borders in the exposed area. Neck skin darkening may also be seen in post-inflammatory hyperpigmentation and other diseases. If the symptoms of darkening of the neck skin, it is recommended that the patient go to the regular hospital in time for a clear diagnosis, and follow the doctor’s instructions to standardize the treatment.