Large subcutaneous bleeding of the legs may be caused by the lack of vitamin C in the body, blood diseases, capillary expansion, leg trauma and other reasons. 1. Lack of vitamin C: If you have been partial to food for a long time or picky about food, resulting in malnutrition in the body, it may lead to a lack of vitamin C in the body, causing the thinning of blood vessel walls, which may lead to the symptoms of massive subcutaneous bleeding of the legs. 2. Blood diseases: If you suffer from blood diseases, such as thrombocytopenia or leukemia, it may lead to blood vessel damage, resulting in massive subcutaneous bleeding of the legs. 3. Capillary dilatation: prolonged sitting, standing or when the legs are stimulated, it may produce blood stasis or even cause brittle rupture of capillaries, which may cause extensive subcutaneous bleeding of the legs. 4. Leg trauma: large subcutaneous hemorrhage can occur when the leg is hit by external force, which can be seen as large petechiae on the leg. It is recommended to go to the hospital for examination to clarify the cause of the disease and timely treatment.