Residual urine may be caused by physiologic or pathologic reasons. 1. Physiological reasons: residual urine is the amount of urine remaining in the bladder after a complete micturition. Under normal circumstances, it is not necessary to empty the bladder every time you urinate, and normal people can have <10ml of residual urine. 2. Pathological causes: generally residual urine volume >30ml suggests that it is caused by pathological causes, including benign prostatic hyperplasia, urethral stones, bladder neck tumors and other mechanical obstructive lesions, as well as spinal cord injuries, diabetic autonomic neuropathy caused by neurogenic bladder and other power obstructive lesions. If the residual urine volume >50ml suggests the existence of urinary retention, >300ml suggests that it may lead to hydronephrosis. Patients are advised to consult a specialist at a regular hospital and follow the doctor’s instructions for further examination to identify the cause of residual urine volume and active treatment.