Retrograde intrarenal surgery (RIRS) has been considered as the first-line choice for the management of <20mm kidney stones. Insertion of a ureteral access sheath (UAS) before RIRS surgery is currently accepted as an effective method to improve the effectiveness of surgery, but can be accompanied by serious complications. In long-term clinical practice, the investigators has found that different Bladder Filling Degree was accompanied by different Resistance During UAS Insertion Procedure in RIRS, which leads to the difference in success rate of UAS insertion.The investigator aims to perform a prospective and randomized controlled trial comparing the success rate of UAS insertion procedure under different bladder filling degree,and discuss the relationship between insertion resistance and ureter injury.

Upper urinary calculi is a common disease that endangers human health, Retrograde intrarenal surgery (RIRS) has been considered as the first-line choice for the management of <20mm kidney stones. Insertion of a ureteral access sheath (UAS) before RIRS surgery is currently accepted as an effective method to improve the effectiveness of surgery, for UAS can drain irrigation to reduce renal pelvic pressure and maintain vision clear. But incorrect way to insert UAS can be accompanied by serious complications. For instance, If the ureter was not dilated by DJ stent before RIRS surgery, or ureter was relatively narrow, the insertion of UAS would encounter great resistance. Sometimes the ureter was violently pushed into UAS, it might cause ureteral perforation or avulsion. As for how to insert UAS correctly, there is no unified international standard at present. Different surgeons have different experience. In long-term clinical practice, the investigators has found that different Bladder Filling Degree was accompanied by different Resistance During UAS Insertion Procedure in RIRS, which leads to the difference in success rate of UAS insertion. In addition, there seems to be some relationship between the resistance of UAS insertion and the ureter injury in surgery. Therefore, the investigator aims to perform a prospective and randomized controlled trial comparing the success rate of UAS insertion procedure under different bladder filling degree, and discuss the relationship between insertion resistance and ureter injury.

Inclusion Criteria:

1. Patients must be a suitable operative candidate for RIRS
2. Age 18 to 70 years
3. Normal renal function 4 .ASA score Ⅰ and Ⅱ

5. Single renal or ureteral stone ≤20mm or multiple stones the conglomerate diameter (additive maximal diameter of all stones on axial imaging of computed tomography) up to 20 mm

Exclusion Criteria:

1. Uncorrected coagulopathy and active urinary tract infection (UTI)
2. Severe cardiopulmonary dysfunction, can not tolerate sugery
3. Patients who underwent transplant or urinary diversion
4. Definite diagnosis of ureteral stricture or a history of stricture
5. The bladder volume is less than 100ml