Bruising on the legs of small children may be caused by trauma, or by allergic purpura, thrombocytopenic purpura and other diseases. 1. Trauma: If the child has collision, fall and other traumatic injuries in daily activities, it will lead to rupture of the child’s capillaries, which will lead to the appearance of bruises on the child’s legs. 2. Anaphylactic purpura: The pathogenesis and etiology of anaphylactic purpura are not yet clear, and it is considered to be caused by bacterial, viral and other pathogenic infections, immune disorders, genetics and other factors. The children will have skin purpura, arthralgia, abdominal pain and so on, which will lead to the phenomenon of bruises on the legs. 3. Thrombocytopenic purpura: Thrombocytopenic purpura is mainly caused by factors such as immunity, heredity, and primary diseases, etc. The main symptoms of the affected children are spontaneous bleeding from the skin and mucous membranes, bleeding from the nasal cavity, and headache. In addition, children with thrombocytopenic purpura will also have visual impairment, disorders of consciousness and other symptoms, which can also lead to the phenomenon of children with bruises on the legs. Bruising on the legs of children may also be caused by leukemia and other diseases, parents are advised to take their children to the hospital in time, and follow the doctor’s instructions for examination and treatment.