Congenital hydronephrosis is one of the common diseases in pediatric urology. Some children with hydronephrosis can be relieved or subside with growth and development, but there are still many hydronephrosis that cannot heal on their own. Because the existence of hydrocele state does not mean that obstruction exists, the biggest doubt for parents is whether their baby should have surgery. For the urologist, it is to identify whether the obstruction will cause permanent damage to the kidney and thus to intervene clinically. In recent years, with the widespread use of fetal ultrasound and the improvement of diagnostic level, fetal hydronephrosis can be detected at 17-18 weeks of gestation during prenatal examination, so the clinical observation of hydronephrosis is significantly earlier. Most pediatric hydronephrosis is caused by obstruction of the pelvic-ureteral junction, posterior urethral valves, ureteral cysts, vesicoureteral reflux and other causes, while dilatation of the pelvis and calyces due to obstruction of the pelvic-ureteral junction is the most common cause, accounting for about 90% or more. The degree of hydronephrosis in children is classified into 5 grades according to the anterior and posterior diameters of the renal pelvis: grade 1 (pelvis dilated <1.0 cm); grade 2 (pelvis dilated 1.0-1.5 cm); grade 3 (pelvis dilated >1.5 cm, calyces mildly dilated); grade 4 (pelvis significantly dilated >1.5 cm, calyces moderately dilated); grade 5 (pelvis and calyces severely dilated with renal parenchymal thinning). At present, it is considered that the indications for surgery in children with hydronephrosis grade 3 or above are clear, but different surgical procedures should be used to treat the children according to the different causes of hydronephrosis.