If you have a nail black line, you can take the following methods of treatment: 1. If you are a child, in most cases it can disappear completely on its own and does not require extra treatment. 2. If you are an adult, you need to observe and follow up first, because it is difficult for adults to disappear on their own after this disease appears. If the nail black line is more than 3mm, you need to be highly alert and take a longitudinal excision, in which all the nails are removed from the lower end of the nail and then the remaining nails are sutured together. Once the nail black line appears, you should go to the hospital for examination and treatment in time to avoid deterioration.