Objective: To investigate the risk factors of fever after the treatment by using FUL. Methods: The clinical data of patients with upper urinary tract calculi who received flexible ureteroscopic lithotripsy (FUL) therapy in First Affiliate Hospital of Harbin Medical University during Jan. 2014 and Dec. 2016 were collected, the indexes of age, gender, the urinary tract infection, hydronephrosis, operation time, diabetes mellitus, the size and location of the stone, irrigation fluid volume, postoperative drainage situation were analyzed with univariate method, then logistic multifactor regression analysis were performed in which have statistically significant index line. Results: Among 122 cases, 26 (21.3%) had a fever after FUL, univariate analysis showed that the urinary tract infection, diabetes mellitus, operation time, the size of the stone, irrigation fluid volume and postoperative drainage situation had statistics significance (P<0.05), and other factors, including age, gender, stone location and hydronephrosis had no statistics significance (P>0.05). Logistic multifactor regression analysis showed that the urinary tract infection, operation time, the size of the stone, irrigation fluid volume and postoperative drainage situation had statistics significance. Conclusion: The postoperative fever rate is higher in patients with preoperative urinary tract infection, so the antibiotics should be actively used during the whole perioperative period; for the patients with large stones or long forecast operation, changing the surgery method or secondary surgery, controlling the operation time and intraoperative low-pressure perfusion may help reduce the rate of postoperative fever.