Retrograde flexible ureteroscopy (RIRS) is currently the first-line treatment for renal stones \< 2cm. Lower pole renal stones(LPS) are a difficult problem for urologists. The flexible negative pressure suction ureteral sheath(f-UAS) can facilitate RIRS to flush out the fragments and dust in time, and provide a clear vision and reduce the renal pelvis pressure(RPP) during operation.Standard lithotomy position is the most commonly used position for RIRS. Besides, T-tilt position is also available for RIRS in special cases. Investigators were inspired by this and proposed the lateral position, which is available in cases of LPS.In long-term practice, investigators have found that the change of position and the use of f-UAS can improve stone-free rate(SFR). Investigators aimed to conduct a prospective randomized controlled trial to compare the SFR of different positions and different ureteral sheaths.