The cause of black lines on the nails is related to the impact of heavy objects, onychomycosis and other factors. 1. Striking with heavy objects: some patients may have subcutaneous bruising when heavy objects strike the nail surface, which will lead to the appearance of black lines on the nails. 2. Nail nevus: the main clinical feature is nail black line, only a very small chance will be transformed into melanoma, and the formation of patients with nail nevus, consider and fungal infection, eczema-like changes, subnail hemorrhage, as well as chemotherapeutic drugs and other factors related. It is suggested that patients should pay attention to the symptoms of black lines on the nails, can go to the dermatology department of the hospital, cooperate with the doctor to improve the relevant examination, to determine the cause, and then do the relative treatment and treatment, in order to avoid delaying the condition.