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 many ureteral injuries occur during UAS insertion of the ureter from ureteral orifice. And if we use rigid ureteroscopy to insert the UAS under direct vision, we can provide the placement success rate and reduce the incidence of complications. The investigator aims to perform a prospective and randomized controlled trial comparing the safety and efficacy of under direct vision and under non direct vision during the insertion of UAS.

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 many ureteral injuries occur during UAS insertion of the ureter from ureteral orifice. And if we use rigid ureteroscopy to insert the UAS under direct vision, we can provide the UAS insertion success rate and reduce the incidence of complications.

Options for the UAS insertion include inserted under fluoroscopy control or inserted by the experience of the surgeon. The former is more commonly used in the United States, while the latter is more commonly used in China. But neither is perfect. Moreover, Ibrahim Karabulut et al. and Mehmet Giray Sönmez et al. reported a different technique for placement ureteral access sheath called "Endovisional technique" in which the outer sheath of UAS was worn on the semirigid endoscope and placed into the ureter under direct vision. Nevertheless, They found that complication rate was lower in the patients who had the UAS placed under "Endovisional technique" when compared to the classical technique. But the difference was not statistically significant. In addition, the investigators are concerned that failure to use sheath cores may cause new damage to the ureter.

So the investigators find a new way to insert UAS without damaging the ureter. In our new procedure, the patient is placed in the lithotomy position, and a 0.035'' flexible tip guidewire is placed into the renal pelvis using a ureteroscope. Then take out the ureteroscope and insert the ureteroscope into urinary bladder beside the guidewire. A 12 Fr/ 14 Fr ureteral access sheath (UAS) is advanced into the ureteral over the guidewire under direct vision. When the UAS successfully enters the ureteral orifice a few centimeters, the ureteroscope is removed. Continue inserting the UAS into the proximal ureter or UPJ and use the direct urinary system X-graphy (DUSG) to confirm that the UAS is inserted into the correct position. A P5 or P6 Olympus flexible ureteroscope is passed through the UAS to finish lithotripsy.

Until now, routine technique to insert ureteral access sheath during RIRS is still under discussion. The investigator aims to perform a prospective and randomized controlled trial comparing the safety and efficacy of under direct vision and under non direct vision during the insertion of UAS.

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 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. Patients with solitary kidney
2. Uncorrected coagulopathy and active urinary tract infection (UTI)
3. Prior ipsilateral endourological procedure history, such as RIRS, PCNL, URS and URL
4. Patients who underwent transplant or urinary diversion.
5. Congenital abnormalities.