To determine the severity of hydronephrosis can be examined by ultrasound, the renal collecting system separation and expansion of 2~3cm for mild hydronephrosis, separation of about 3~4cm for moderate hydronephrosis, the degree of separation is greater than 4cm for severe hydronephrosis. 1. Mild hydronephrosis: under ultrasound examination, it is found that the thickness of renal parenchyma and renal echogenicity are normal, and the renal collecting system is separated and expanded by 2~3cm, and at this time, anechoic area can be seen in the renal sinus. It is usually due to poor rotation of the kidney or stone obstruction, etc., which leads to obstruction of renal urination and causes corresponding symptoms. 2. Moderate hydronephrosis: the renal collecting system is separated by about 3~4cm, the volume of kidney is slightly enlarged, and the renal pelvis and calyx are separated in the shape of a pipe. Patients with ureteral stenosis, stones, tumors, etc., will lead to ureteral compression and dilatation of renal pelvis and renal calyces, causing moderate hydronephrosis. 3. Severe hydronephrosis: the renal collecting system is separated by more than 4cm, showing liquid dark area, and the echoes of the renal column can be seen arranged in a radial pattern, and the renal cortex is obviously thinned. Usually caused by the patient’s renal ureteral connection stenosis. Determination of the severity of hydronephrosis can generally be distinguished by ultrasound, the specific operation needs to be carried out under the guidance of a doctor.