The swelling of the back of the foot may be caused by trauma, blood disease, purpura and other reasons. 1. Trauma: Trauma is a common cause of swollen and purple dorsum of the feet, the dorsum of the feet after the injury, will cause subcutaneous blood vessel damage, subcutaneous bleeding, and cause the dorsum of the feet swollen and purple. 2. Blood diseases: such as thrombocytopenia, hemophilia, etc., these diseases will cause coagulation dysfunction, which will cause the swelling of the back of the feet greenish-purple, but also lead to other parts of the skin bleeding. 3. Purpura: such as purpura simple, allergic purpura, etc. Purpura simple refers to the absence of other diseases, in the skin, especially in the lower limbs of the spontaneous recurrent bleeding disease. Allergic purpura refers to the body’s allergic reaction to certain sensitizing substances, resulting in increased capillary fragility and permeability, which causes inflammation and bleeding of small blood vessels in the skin, joints and other parts of the body. It can be seen that the cause of the swelling of the back of the foot blue purple cause is very much, such as acute trauma caused by or repeated spontaneous appearance, you should go to the hospital in time for further diagnosis and treatment.