The appearance of purple ecchymosis on the arm may be related to blood diseases, trauma, adverse drug reactions and other reasons. 1. Blood diseases: blood diseases such as idiopathic thrombocytopenia, due to excessive destruction of platelets leading to a decrease in platelets in the peripheral blood, which can cause spontaneous bleeding under the skin, manifested by purple bruises on the arms. 2. Trauma: If the arm is hit by external injuries, it can also cause subcutaneous bleeding and localized bruising, usually with a history of trauma and accompanied by obvious pressure pain. 3. Adverse drug reactions: such as aspirin, its common adverse reactions are spontaneous bleeding, peptic ulcer, etc., can appear on the arm with the phenomenon of bruises. It is recommended that people with purple petechiae on the arm go to the regular hospital in time, to clarify the cause of the disease, should be standardized treatment under the guidance of a specialist.