The causes of black lines on the nail cover include trauma, onychomatous nevus, melanoma and other factors. 1. Trauma: when nails are damaged by external force, black lines can appear after blood coagulation due to bleeding under the nails, this black line can be absorbed gradually with time and no special treatment is needed. 2. Nail nevus: due to genetic and environmental factors, nevus cells produce melanin and deposit it on the newborn nail plate, at this time, the phenomenon of black lines on the nail cover can appear, and it may be a nevus of the nail. 3. Melanoma: If various factors lead to the abnormal proliferation of melanocytes, then the symptom of black line will also appear, if the black line widens fast, the color is uneven, there may be bleeding or ulcer phenomenon, it may be melanoma, and this situation needs to consult a doctor in time. If there is a black line on the nail cover, it is recommended to go to the hospital in a timely manner to clarify the cause of the disease and treat under the guidance of the doctor.