Darkening of the skin on the lower back of men may be caused by physiological factors such as over-tightening of the waistband and friction, as well as pathologic factors such as subcutaneous bleeding, post-inflammatory hyperpigmentation and acanthosis nigricans. 1. Physiological factors: if the belt is too tight, it affects blood circulation, which may lead to the blackening of the skin of the back of the waist, which can be aggravated by friction. In addition, if the male back waist skin is exposed to sunlight for a long time, it can lead to a large amount of melanin deposition, and the problem of skin blackening can also occur. 2. Subcutaneous hemorrhage: If the male posterior waist subcutaneous hemorrhage, can lead to pigment deposition in the dermis, the appearance of dark brown patches, there will be a male posterior waist skin darkening problem. 3. Post-inflammatory hyperpigmentation: After inflammation of the skin, it may lead to melanin proliferation and darkening of the skin at the lower back. 4. Acanthosis nigricans: It is a rare skin disease mainly characterized by excessive skin pigmentation, so darkening of the skin at the lower back in men may be caused by acanthosis nigricans. If the skin of the lower back is dark, after excluding physiological factors, it is recommended to consult a doctor in time to clarify the cause of the disease under the guidance of the doctor and timely treatment.