Bruising may be caused by trauma causing subcutaneous hematoma, or skin purpura caused by various diseases, such as allergic purpura, primary thrombocytopenic purpura, hereditary hemorrhagic capillary dilatation, etc.. 1. Trauma: soft tissue contusion and capillary rupture due to external impact, etc., resulting in subcutaneous hematoma, causing bruises on the legs. 2. Anaphylactic purpura: the formation of small blood vessels of IgA class immune complexes in the body due to allergy to certain drugs or food, etc., resulting in endothelial damage to the blood vessels in the skin and other parts of the body. Treatment should avoid contact with allergens, at the same time to pay attention to rest and strengthen nutrition, but also follow the doctor’s instructions to use antihistamines, glucocorticoids, immunosuppressants and other drugs. 3. Primary thrombocytopenic purpura: the cause of this disease is not yet clear, may be due to viral infections and other causes of their own production of anti-platelet antibodies, resulting in a reduction in the number of platelets or weakening of the function of platelets, which causes the distal extremities of the skin ecchymosis. Treatment can be systematic application of glucocorticoids, immunosuppressants and other drugs as prescribed by the doctor, or splenectomy can be chosen under the guidance of the doctor. 4. Hereditary hemorrhagic capillary dilatation: the disease is hereditary, due to increased mechanical fragility of blood vessels, dilated capillaries are more prone to rupture, leading to subcutaneous hemorrhage, resulting in bleeding and bruising of the legs and other phenomena. Treatment of this disease requires blood transfusion if severe anemia occurs; in severe cases of gastrointestinal bleeding, endoscopic hemostasis is required. Frequent bruises on the legs may also be caused by other diseases, it is recommended that patients go to the regular hospital in a timely manner, after a clear diagnosis under the guidance of the doctor according to the cause of the treatment, so as not to delay the condition.