The appearance of a ring of black on the penis may be caused by physiological factors, excessive masturbation stimulation or penile melanoma. 1. Physiological factors: men who masturbate excessively or have sex frequently will cause frequent stimulation of the skin on the penis, pigmentation, thus making the penis appear a circle of black, usually not accompanied by other discomforts, is a normal physiological phenomenon. 2. Excessive masturbation stimulation: patients in masturbation, too much force, will make the penis by excessive pressure, resulting in poor blood circulation, so that the penis appears a circle of black, but also with swelling, pain and other symptoms. 3. Penile melanoma: a circle of black penis may also be caused by penile melanoma. The symptoms of penile melanoma are manifested in the penile area, which can be accompanied by black irregular lumps, and as time goes by, the color will turn into red, or even black-red. When the patient’s penis appears a circle of black, if accompanied by other discomfort, it is recommended to go to the hospital in time.