The condition of black spots on arms is considered to be related to congenital nevus, seborrheic keratosis, post-inflammatory hyperpigmentation and other factors. 1. Congenital nevus: some patients may have the phenomenon of black spots on arms when they are born because of congenital hereditary factors, if there is no other symptom, there is no need for special treatment. 2. Seborrheic keratosis: it belongs to common epidermal benign tumors, especially for adults over 40 years old, because of skin aging as well as genetic susceptibility, etc., which will cause patients to have localized plaques of varying sizes, which may be brown or black. 3. Post-inflammatory hyperpigmentation: it belongs to acquired pigmentation abnormality disease, especially after acute and chronic inflammatory process of the skin, it will be followed by hyperpigmentation phenomenon of the skin, so it will cause the manifestation of long black spots on the arms. When the patients have long black spots on the arms, which may be related to other diseases, it is recommended to go to regular hospitals for consultation and choose appropriate treatment plan after clear diagnosis as suggested by specialized physicians.