Hydronephrosis is a condition caused by the retention of urine in the renal pelvis, which cannot be discharged smoothly through the ureter. Hydronephrosis is only a manifestation caused by certain diseases, and the specific causes of hydronephrosis need to be clarified in order to treat it effectively. The main causes of bilateral hydronephrosis at the same time are obstruction of the lower urinary tract or bilateral ureteral obstruction. The specific conditions are as follows. If there is bilateral hydronephrosis at the same time, the first thing to consider is lower urinary tract obstruction. The more common causes are urinary retention caused by prostatic hyperplasia in elderly men, urethral stones, huge stones in the bladder, tumors compressing the ureteral opening and other conditions blocking the ureteral opening and urethra, resulting in urine not being discharged smoothly and causing bilateral hydronephrosis, which is mostly accompanied by bilateral ureteral dilatation. Most of the cases can be clearly diagnosed through ultrasound examination, and the hydronephrosis can be relieved by restoring the patency of the lower urinary tract through indwelling catheter or removing urethral stones. In addition, upper urinary tract obstruction caused by bilateral ureteral stones and bilateral kidney stones can also cause bilateral hydronephrosis. In this case, lithotripsy treatment is needed to restore upper urinary tract patency to resolve bilateral hydronephrosis. Secondly, due to the increase in the size of the uterus of women in the second trimester of pregnancy, compression of the ureter can also cause hydronephrosis, which will be relieved after delivery by lifting the compression of the ureter. In summary, the main cause of hydronephrosis is lower urinary tract obstruction, but it can also be seen in bilateral upper urinary tract obstruction at the same time, in addition, diseases outside the urological system can also cause hydronephrosis, it is necessary to carefully identify the causes of hydronephrosis through examination.