Moles, also known as pigmented nevi or melanocytic nevi, are benign skin tumors of melanocytic origin. The cause of new nevus on the arm is not completely clear, but it can be related to immune system disorder, ultraviolet radiation, hormone level change and so on. 1. Immune system disorder: due to various factors leading to immune system dysfunction, the immune system will attack the normal skin and trigger the appearance of new nevus. 2. Ultraviolet radiation: When the skin is stimulated by ultraviolet rays for a long time, the ultraviolet rays will damage the skin and cause new moles to appear. 3. Hormone level change: when the hormone level changes during pregnancy, the hormone stimulation will trigger the proliferation of skin tissues, and the phenomenon of new moles on the arm will also occur. There may be other reasons for the growth of new moles on the arms, which should be examined and treated in a standardized way.