There are many causes of small red dots on the arm, such as cherry angioma, purpura, perikeratosis, etc. 1. Cherry hemangioma: it is a benign hemangioma, which increases with age, the cause of which is still unclear, causing the appearance of small round or ovoid papules on the skin, with a diameter of about 0.1~0.4 cm, soft to the touch, no pain when pressed, and the color is mostly red, which turns into a normal skin color when pressed. It can cause the appearance of small red dots on the arms. 2. Purpura: for example, allergic purpura, idiopathic thrombocytopenic purpura, can cause subcutaneous capillary rupture, resulting in skin bleeding spots, subcutaneous petechiae and other symptoms, may also cause the appearance of small red dots on the arm. 3. Keratosis pilaris: mostly related to heredity and metabolic disorders. Clinically, it is mainly manifested as dense follicular papules on the upper and lower limbs, cheeks, skin color, red, etc., occasionally itchy. It can also cause small red spots on the arms. Small red dots on the arm, there may be other reasons, it is recommended to go to the hospital in time to check, clarify the cause of the disease and follow the doctor’s instructions for treatment.