Purple spots on the back of the foot may be caused by allergic purpura, thrombocytopenic purpura, or small skin vasculitis, which need to be analyzed according to the symptoms. 1.allergic purpura: an IgA antibody-mediated allergic capillary and small vessel vasculitis, preferably in the lower limbs, before the onset of the onset of upper respiratory tract infections, low-grade fever, general malaise and other prodromal symptoms, followed by the appearance of red pimples, usually within 24 hours of the development of the tip of the needle to the size of the soybean purpura, palpable as a characteristic of the ecchymosis can be also petechiae. 2. Thrombocytopenic purpura: Thrombocytopenic purpura is a hemorrhagic disease characterized by bleeding of the skin and mucous membranes due to thrombocytopenia. Thrombocytopenic purpura is mainly related to immune factors, genetic factors or primary factors. Thrombocytopenic purpura may be manifested as purple spots on the back of the feet, etc. In severe cases, multiple organ or intracranial hemorrhage may occur. 3. Cutaneous small vessel vasculitis: refers to the vasculitis that simply involves the small dermal blood vessels. It occurs in the lower limbs and buttocks, and the lesions are polymorphic, which can be manifested as erythema, papules, purpura, blisters, blisters, vesicles, vesicles, ulcers, necrosis, and superficial nodules, etc., but purpura, ulcers, necrosis, and nodules are the main features. There may be other reasons for the purple spots on the back of the feet, and you should go to the hospital in time to avoid delaying your condition.