Hydronephrosis is a common disease in urology. Mild, moderate and severe hydronephrosis are commonly used clinically to describe the degree of hydronephrosis. Specifically, we can use urological ultrasound and urological enhancement CT to understand the degree of hydronephrosis specifically. The maximum anteroposterior diameter of the renal pelvis is measured in cross-section after urination, normally less than 1 cm. 1.5 cm or less indicates pelvic separation. 1.5 cm or more can be considered as mild hydronephrosis in combination with medical history and other manifestations. 2.0 cm to 3.0 cm can be reported as mild hydronephrosis. In moderate hydronephrosis, the pelvis and calyces are more significantly dilated. In severe hydronephrosis, the separation of the collecting system is often greater than 4 cm, the volume of the kidney increases, the renal parenchyma is significantly thinner or cannot be displayed, and the kidney area is liquid and dark, radially arranged, in the shape of a “palette”. It should be noted that the severity of hydronephrosis is not consistent with the extent of the lesion and the damage to the kidney function, so clinical diagnosis and treatment need to be considered comprehensively.