The desire to urinate on erection may be a normal physiological phenomenon, but it may also be caused by prostatitis, urethritis and other reasons. 1. Normal physiological phenomenon: when the penis is erect and the bladder is full, the engorged corpus cavernosum will easily stimulate the mucous membrane of the urethra, which will produce the desire to urinate, resulting in the patient wanting to urinate on erection. 2. Prostatitis: If the patient has prostatitis, in the case of little water, the prostate gland after erection of the prostate gland congestion, will stimulate the prostate gland, will also lead to the emergence of the desire to urinate, at the same time, may be accompanied by frequent urinary urgency. 3. Urethritis: Urethritis itself can lead to frequent urination. It may be that after the penis is erected, the urethritis stimulates the urinary tract and bladder, leading to the condition of wanting to urinate when erected. Patients in the exclusion of non-physiological causes, should promptly go to the regular hospital to identify the cause of the disease, under the guidance of professional doctors to give targeted treatment.