A black line in the middle of the index finger may be caused by trauma, malnutrition, onychomycosis and other factors. If the index finger area is accidentally hit by a heavy object or pinched, it may cause localized bleeding in the index finger, leading to capillary rupture, which causes subnail bruising, and the bruising may form a brown or black line. It is recommended to treat the injury promptly with alcohol or iodophor. If there is picky eating or improper diet, partiality and other bad habits in daily life, it may cause malnutrition, which may cause insufficient vitamin intake and local lesions, resulting in the above situation. At this time, it is necessary to improve one’s dietary habits in time to ensure adequate nutrition, and the symptoms will gradually disappear with the improvement of physical condition. Mole belongs to a kind of mole, hemorrhoids may grow at the root of nails, and with the growth of nails, gradually form a gray or black line, and may even turn into melanoma, it is recommended to go to the regular hospital in time for checkups, and if you suffer from nevus of nails, you should treat it through nail extraction in time.