Black patches on the legs are considered to be related to impact of foreign objects, thrombocytopenic purpura, nevus and so on. 1. Foreign object impact: when the patient’s legs suffered foreign object impact, resulting in local capillary rupture bleeding, bruising, may cause the phenomenon of black plaque on the legs. 2. Thrombocytopenic purpura: the cause of thrombocytopenic purpura is not clear, may be related to genetic factors, immune disorders. The disease may cause spontaneous bleeding of the skin and mucous membranes, which is manifested as black patches on the legs. 3. Nevus: Melanoma can also cause some patients to have black patches on the legs. The etiology of nevus is not clear, it may be related to ultraviolet radiation, family history, etc. When accompanied by itching, bleeding, ulceration, pain and other symptoms, there is a possibility of malignant transformation, and it should be completely excised in time and sent to do pathological examination. It is recommended that people with black plaques on the legs should consult a doctor for examination, and if it is clear that the disease is caused by the disease, they should actively cooperate with the doctor for treatment.