Objective To explore the causes of reoperation after pyeloplasty for ureteropelvic junction obstruction (UPJO) in children. Methods Clinical data of pediatric patients undergoing reoperation after pyeloplasty for UPJO from January 2015 to December 2017 were collected. The reasons of reoperation were analyzed. Results A total of 36 UPJO children underwent reoperation after pyeloplasty. Anastomotic edema, adhesion and fibrotic scar around the stoma was found in 26 (72.22%) patients. Other reasons were polypus near the anastomotic stoma (5 cases, 13.89%), high insertion of the ureteropelvic junction (5 cases, 13.89%), aberrant vessels compressing the ureter (3 cases, 8.33%), stenosis of distal ureter (2 cases, 5.56%), and renal calyx atresia (2 cases, 5.56%). Two patients who were diagnosed as having renal calyx atresia underwent renal calyx plasty and lower calyceal ureteral anastomosis. The other patients received open pyeloplasty. The median follow-up time was 28 months after reoperation. During the follow-up, 34 patients experienced no restenosis of ureteropelvic junction. But one patient suffered from second restenosis, and underwent renal calyx plasty and lower calyceal ureteral anastomosis again. One patient experienced abdominal pain and aggravation in hydronephrosis 30 months after the reoperation. He was diagnosed as having cicatricial stenosis after the third pyeloplasty. The two patients had no stenosis after follow-up for 37 and 20 months, respectively. Conclusion Anastomotic edema, adhesion and fibrotic scar are the main causes of reoperation after pyeloplasty. Other rare but important causes are iatrogenic fibroepithelial polyps, high insertion of the ureteropelvic junction, aberrant vessel compression, stenosis of distal ureter, and renal calyx atresia. During the operation, it is necessary to perform accurate anastomosis in spacious space of the ureter, keep the anastomotic stoma at the low renal pelvis, avoid the injury of renal calices and the ignorance of the aberrant renal vessels, and keep the head of the fistula as far as possible from the anastomotic stoma. These may help to reduce the possibility of reoperation after pyeloplasty.