A purple bruise on the child’s leg may be caused by idiopathic thrombocytopenic purpura, anaphylactic purpura and other causes. 1. idiopathic thrombocytopenic purpura: idiopathic thrombocytopenic purpura may be related to heredity, infection, immune factors, etc. More than half of the patients are children, the main manifestations are petechiae, violet spots, ecchymosis and so on on the surface of the skin, and some of the patients may also have symptoms such as bleeding from the oral mucosa and nose. 2. allergic purpura: allergic purpura may be related to heredity, infection, immune disorders, allergen stimulation and other factors, sufferers of the most common in children, preferably in the limbs and buttocks, the rash at the beginning of the purplish-red papules higher than the plane, do not fade when pressed, and after a few days to change into a dark purple. A purple bruise on the child’s leg may also have other causes, it is recommended to go to the hospital in a timely manner to improve the examination, to clarify the cause of the disease to give targeted treatment.