Small red spots on the arm may be caused by cherry angioma, allergic purpura, spider nevus and other diseases. 1. Cherry hemangioma: the specific cause of cherry hemangioma has not been fully clarified, is due to the abnormal proliferation of vascular endothelial cells, resulting in vascular hyperplasia. The typical symptom of the disease is the appearance of deep red, painless papules on the trunk skin, which can occur in the arm. 2. Anaphylactic purpura: The cause of anaphylactic purpura is not yet very clear, and it is considered to be related to systemic dysfunction, autoimmunity, heredity, infection and other risk factors. Rash caused by purpura is similar to bleeding dots, slightly above the skin surface, sometimes fused into patches, and does not fade when pressed. 3. Spider nevus: the exact cause of spider nevus is not clear, and it is considered to be related to the increase of estrogen level. Spider nevus is a kind of capillary dilatation, which can occur on the neck, face, chest, arms, fingers, back of hands and other parts. 4. Psoriasis: the disease is an inflammatory disease caused by the interaction of genetic, environmental and immune factors, and clinically it mainly manifests as scaly red plaques accompanied by itching, and there are also small red spots on the arms. Small red spots on the arms are considered to be related to the above factors. It is recommended to go to the hospital for examination, ask the doctor to make a diagnosis, and then develop an appropriate treatment plan.