Black lines on thumb nails may be caused by onychomatous nevus or traumatic injury, or it may be the activation of melanocytes in the nails. 1. Nevus of Onychomycosis: the cause of nevus of onychomycosis is not clear at present, the main symptom is that there will be darker color and clearer black line on the nail, which is distributed in the form of strips and bands; 2. Trauma: If the thumb is traumatized, bruising occurs, and the blood coagulates, it will also appear as a black line on the thumb nail, which usually recovers on its own without special treatment; 3. Nail melanocyte activation: black lines on the thumb nail may also be nail melanocyte activation, due to increased synthesis of nail melanocytes, the main manifestation is that there will be uniform melanin bands on the nails. Thumb nails have black lines recommended to actively go to the relevant departments of the regular hospital to find out the specific causes, and then targeted treatment, do not be negligent, so as not to delay or aggravate the condition.