The main cause of hydronephrosis is still urinary tract obstruction, the common site of obstruction is at the pelvic ureteral junction, and there are more reasons for obstruction, there may be urinary stones, there may be local tissue congenital dysplasia, there may be local tumor lesion compression and other conditions. I. Congenital causes of hydronephrosis: 1. Intrinsic ureteral stenosis: often occurs at the ureteropelvic junction, producing an incomplete obstruction situation, forming a local inelastic stenotic area, leading to urine accumulation and retention, forming hydronephrosis. 2. Segmental non-functionality: Due to congenital developmental abnormalities, it leads to local loss of ureteral contraction function and formation of obstructive conditions. 3.Ureteral distortion, adhesions, abnormal structures, etc., leading to local obstruction and urine retention. 4, Ectopic vascular compression: for example, lesions such as horseshoe-shaped kidney will have ectopic vascular compression of the ureter, leading to retention situation. 5, Ureteral opening is too high, which may also lead to increased local tension and formation of urine retention. Second, acquired causes of hydronephrosis: 1, inflammatory stimulation: long-term chronic inflammatory stimulation may lead to the formation of scar tissue inside the ureter, resulting in abnormal contraction function and obstruction. 2, the junction of the renal pelvis ureter if there is stone obstruction: tumors, neoplastic and other tissues, can also be compressed leading to obstruction situation. 3, trauma factors: if there is damage to the ureter, in the repair process, there will be scar tissue formation, which will also affect the contraction function of the ureter, leading to abnormal function of the situation of hydrocele. Regardless of which situation leads to hydronephrosis, once found, it is still recommended to promptly check and treat the cause of hydronephrosis on the one hand, and on the other hand, according to the examination, choose the appropriate surgery to avoid the aggravation of the condition leading to kidney infection.