Small red dots on the arm may be caused by physiological reasons, or may be caused by pathological reasons such as cherry angioma, atopic dermatitis and allergic purpura. 1. Physiological reasons: if the arm is subjected to prolonged stimulation (such as washing clothes, outdoor sun exposure, etc.), some small red dots will appear, avoid contact with the stimulus, after a period of time can disappear. 2. Pathological causes: (1) Cherry angioma: it is a common benign skin vascular proliferative disease with unknown etiology, mostly occurring in the trunk and both upper limbs, which can be manifested as scattered red dots on the arms. (2) Allergic dermatitis: common urticaria, eczema, etc., urticaria manifested as erythema in many parts of the body, doughnut-like rashes, eczema to repeated episodes of erythema, papules, blisters, are likely to appear on the arm long red dots. (3) Anaphylactic purpura: caused by various irritating factors (e.g. allergens, infectious agents, etc.), it is a hemorrhagic disease manifested by scattered ecchymotic bruises on the skin. There are many causes of red dots on the arm, and it is recommended to consult a doctor for a clear diagnosis so as not to delay the condition.