Bruising on the legs for no reason may be related to trauma and blood system diseases. 1. Trauma and bumping: external force can lead to the rupture of capillaries in the legs and the formation of bruises with blood accumulation under the skin, but the bruises can be gradually absorbed and disappear after a few days. 2. Blood system diseases: such as primary immune thrombocytopenia patients, the examination of the distal extremities skin purpura or bruises, visible nosebleeds, gum bleeding and other mucosal bleeding performance; coagulation disorders such as vascular hemophilia, nosebleeds, skin bruises and other manifestations. Legs appear unexplained bruises, it is recommended to immediately go to the hospital specialist consultation, coagulation function, blood routine and other related examinations, clear cause after targeted treatment.