Children’s toenails black may be physiological, may also be pathological factors such as trauma, nail fungal disease and other causes. 1. Physiological factors: such as children do not pay attention to long-term toenail hygiene, infrequent cleaning, etc. can be caused by black toenails, is a normal phenomenon, timely and good cleaning symptoms can be alleviated disappear. 2. Pathological factors (1) Trauma: such as external injuries, extrusion, etc. caused by localized bruising of the toenail, the symptoms of blackening. (2) Nail fungus disease: when the toenail is infected with fungus, the fungus can have an invasive effect on the nail plate and the subnail tissue, causing nail fungus disease, which is manifested as the symptom of blackened toenail. If children have symptoms of black toenails, it is recommended that they seek immediate medical attention to clarify the cause of the disease and then actively treat the cause, so as not to delay the condition.