Causes of purple bruise spots on the arm include hemangioma, local trauma, abnormal blood clotting function and so on. Since the specific situation of each person is different, it is recommended to go to the hospital for examination, after clarifying the cause of the disease, treat the cause of the disease. 1. Hemangioma: When people suffer from hemangioma, blood vessels will proliferate abnormally and bruises will appear. It is recommended to go to the hospital for clear diagnosis and active treatment. 2. Trauma: after the arm is squeezed by trauma, or after soft tissue contusion, purple bruises will appear on the epidermis. 3. Abnormal coagulation function: When people suffer from blood diseases (such as thrombocytopenic purpura), purple bruises will appear on the arm and other peripheral areas because of abnormal coagulation function. If purple bruises always appear, and exclude traumatic factors, you should consult a doctor in time.