The urinary system is a continuous system of tubes starting from the kidneys and ending at the urethra. The discharge of urine depends on the smoothness of the urinary tract and the normal function of urination. Narrowing or obstruction of any part of the urinary tract and normal neuromuscular dysfunction eventually lead to hydronephrosis, dilatation, thinning of the renal parenchyma, and decreased renal function. There are many causes of urinary inaccessibility (obstruction), which can be congenital or acquired diseases. Acute obstruction causes complete loss of kidney function within a short period of time, while hydronephrosis is not very obvious. Chronic obstruction can cause a gradual increase in hydronephrosis to giant hydronephrosis. Once the hydronephrosis is complicated by infection, if the obstruction is not released in time, the infection is difficult to be cured, and the infection accelerates the destruction of the kidney, forming a vicious circle, leading to renal failure and septic kidney. Hydronephrosis is mainly caused by three types of diseases: 1, ureteral abnormalities: including stenosis, valves, stones, cysts, ectopic openings, giant ureter, etc. This is a common disease causing hydronephrosis, resulting in urine in the kidney not flowing easily to the bladder and stagnating in the kidney. 2, vesicoureteral reflux: This includes primary vesicoureteral reflux and neurogenic bladder. Under normal circumstances, urine reaches the bladder from the kidneys via the ureter and is then expelled from the body; urinary reflux is an abnormal phenomenon in which urine flows backwards from the bladder to the ureter or even to the kidneys. Since the kidneys produce urine at the same time, as well as the urine backflowing from the bladder, the urine in the kidneys will increase at this time, resulting in hydronephrosis. 3 Obstruction of the urethra, generally seen in boys, such as urethral valves and urethral strictures. Treatment: To remove the obstruction based on the elimination of the cause, improve renal function, relieve symptoms, eliminate infection, protect renal function, and repair its normal anatomy as much as possible. However, there is no specific treatment for neurogenic bladder. The management of cases with clear pathological hydronephrosis in the fetus is still controversial, and intrauterine surgery with fetoscopy has been reported abroad to be more damaging to the fetus. For children with hydronephrosis after birth, they should be followed up regularly, and if necessary, urography, CT scan with enhancement, MRI, etc. can be used to assist in the diagnosis. If pathological obstruction is found and the degree of hydronephrosis is moderate or severe, surgery should be performed as soon as possible to prevent further damage to renal function. If the hydronephrosis is mild and does not worsen during the follow-up, surgical intervention is not urgent, but should be observed regularly until the hydronephrosis is reduced or disappears.