The causes of darkening of waist skin include congenital hyperpigmentation, repeated friction, post-inflammatory hyperpigmentation, and acanthosis nigricans. 1. Congenital hyperpigmentation: due to heredity and other factors, melanin deposition of the waist skin is triggered, and the phenomenon of darkening of the waist skin can occur at this time. 2. Repeated friction: when the waist is repeatedly friction, the keratin is thickened and the melanin production increases, which will also lead to the phenomenon of darkening of the skin. 3. Post-inflammatory hyperpigmentation: it refers to the hyperpigmentation secondary to acute or chronic inflammation of the skin, due to the inflammatory response to stimulate some of the cytokines, resulting in melanin proliferation, so the skin is dark. 4. Acanthosis nigricans: mostly seen in obese people and insulin resistance people, manifested as skin roughness, color deepening. Caused by the waist skin black may also have other reasons, it is recommended to consult a doctor in a timely manner to clarify the cause of the disease, in accordance with the doctor’s instructions for reasonable treatment.