The sudden appearance of bruised purple mass on the arm may be triggered by trauma, may be thrombocytopenia, and may also be allergic purpura and other causes. At this time, it is recommended to go to the hospital for differential diagnosis to avoid delaying the condition.1. Trauma factor: If the arm is accidentally hit, due to the external force, resulting in the rupture and bleeding of the subcutaneous capillaries of the arm, it will make the local bruise purple block change. For this type of subcutaneous vascular rupture bleeding, it is recommended to choose cold compresses within 24 hours to promote vasoconstriction and reduce bleeding. 24 hours later, choose hot compresses to promote local absorption of bruises and accelerate healing, do not worry too much; 2, thrombocytopenia: due to the reduction of platelets, coagulation function will decline, prone to damage to the blood vessel wall, subcutaneous bleeding, in the form of dots or flakes, at this time the arm may suddenly appear The situation of bruised purple clots. It is recommended to find the cause of thrombocytopenia, such as aplastic anemia, acute leukemia, etc., and then carry out specific treatment according to the different causes. For example, aplastic anemia can be treated with immunosuppressive drugs as prescribed by the doctor, commonly used drugs include cyclosporine soft capsule, anti-human lymphocyte immunoglobulin and so on. And leukemia for patients with stable condition and eligible for transplantation, they can also undergo hematopoietic stem cell transplantation; 3. Allergic purpura: there are relatively more triggering factors, including food, medicine, infection, etc. In the absence of trauma, dark purple spots or plaques are seen under the skin, but not accompanied by thrombocytopenia, when a sudden bruised purple mass can appear on the arm. Non-steroidal anti-inflammatory drugs can be used as prescribed to relieve pain, commonly used drugs include ibuprofen extended-release capsules, naproxen tablets, etc. It is also possible to use glucocorticoids as prescribed by the doctor, commonly used are methylprednisolone tablets, dexamethasone tablets, etc.; 4, simple purpura: young women are common, the cause is not clear, may be related to female hormone level secretion metabolic disorder, capillary permeability abnormalities, chromosomal abnormalities, etc. The main symptoms include recurrent spontaneous bruises and petechiae on the skin, when a sudden bruised purple mass on the arms can occur, usually without special treatment, but patients should avoid taking non-steroidal anti-inflammatory drugs during the disease, such as indomethacin capsules and ibuprofen extended-release capsules.