The main cause of hydronephrosis is obstruction of the urinary tract, which can be divided into congenital and acquired causes. Congenital causes of hydronephrosis include intrinsic ureteral stenosis, segmental nonfunction, ureteral torsion, ectopic vascular compression, and high ureteral opening. Intrinsic ureteral stenosis is often found at the pelvic-ureteral junction, creating an incomplete obstructive condition that creates a localized inelastic area of stenosis, leading to urine accumulation and retention, resulting in hydronephrosis. Segmental nonfunction is due to congenital developmental abnormalities that result in localized loss of ureteral contraction and the formation of an obstructive condition. Ureteral twisting, adhesions, and abnormal structures lead to local obstruction and urine retention. Ectopic vascular compression can have ectopic vessels compressing the ureter, leading to retention conditions. A high ureteral opening may also lead to increased local tension and urine retention. Acquired causes of hydronephrosis include inflammatory irritation, pelvic ureteral junction obstruction, and traumatic factors. Long-term chronic inflammatory irritation may lead to the formation of scar tissue inside the ureter, resulting in abnormal contractile function and obstruction. If there is stone obstruction, tumor, neoplastic and other tissues at the junction of the renal pelvis and ureter, it may also cause obstruction by compression. If there is damage to the ureter, scar tissue will form during the repair process, which will also affect the contractile function of the ureter, leading to abnormal function and the development of fluid retention.