A black spot on the inside of the toenail cap is considered to be related to trauma, onychomatous mole and subnail melanoma. 1. Trauma: If the toenail is subjected to external injuries such as extrusion or blows, the blood vessels under the toenail will rupture and bleed, resulting in bleeding and bruising under the nail, which will lead to a black spot inside the toenail cap. 2. Nail nevus: a pigmented nevus that occurs on the nail, the cause of which is still unclear. Patients with onychomatous nevus will have black lines on their nails, and usually the black lines are regular in shape and boundary. 3. Subnail melanoma: belongs to malignant tumor of melanocyte origin, the disease is clinically manifested as enlargement of the original mole, color change, unclear border and bleeding. The cause of a piece of black inside the toenail cover is not one, and it is recommended that the patient go to the regular hospital for consultation to clarify the cause of the disease.