The collecting system of the kidney mainly refers to the renal calyces and pelvis. After the kidneys produce urine, it gradually converges to the middle part of the kidneys, first to the renal calyces, then to the renal pelvis, and then to the bladder through the ureters. The renal calyces and renal pelvis cannot be completely distinguished under ultrasound, and the area that appears to be lumpy is called the collecting system. Generally, the collecting system is less than 0.7 cm, and if it exceeds 0.7 cm, especially if it exceeds 1 cm, it is called a separated collecting system. The separation of the collecting system usually indicates hydronephrosis, and long-term hydronephrosis will lead to thinning of the renal cortex under pressure and gradual loss of function. Therefore, after the discovery of hydronephrosis, the next step is to actively search for the cause of hydronephrosis and give treatment. The more common causes of hydronephrosis are urinary stones, pelvic ureteral junction stenosis and tumors of the urinary tract.