This study compares the efficiency and safety of a novel flexible ureteral access sheath (f-UAS) and a traditional ureteral access sheath (UAS) during retrograde intrarenal surgery (RIRS) for treating renal stones. The f-UAS, designed with a 10-cm-long tube at the tip that can follow the bends of a flexible ureteroscope (f-URS), was used in 152 patients, while the traditional UAS was used in another 152 patients. The results showed that the f-UAS group had significantly higher stone-free rates (SFR) at 1 day postoperatively (76.3% vs. 7.2%, P < 0.001) and higher stone volume clearance rates (98.11% vs. 91.78%, P < 0.001). The f-UAS group also had lower total complication rates (9.9% vs. 22.4%, P = 0.003), lower incidence of fever (5.9% vs. 11.9%, P = 0.001), shorter operative times (56.5 min vs. 59.9 min, P = 0.047), and lower usage rates of baskets (17.1% vs. 100%, P < 0.001). However, there was no significant difference in SFR at 1 month postoperatively (P = 0.627) and in the length of postoperative hospital stay between the two groups (P = 0.225). The study concludes that the f-UAS offers several advantages over the traditional UAS, including higher SFR, shorter operative times, lower incidence of complications, and reduced use of baskets.This study compares the efficiency and safety of a novel flexible ureteral access sheath (f-UAS) and a traditional ureteral access sheath (UAS) during retrograde intrarenal surgery (RIRS) for treating renal stones. The f-UAS, designed with a 10-cm-long tube at the tip that can follow the bends of a flexible ureteroscope (f-URS), was used in 152 patients, while the traditional UAS was used in another 152 patients. The results showed that the f-UAS group had significantly higher stone-free rates (SFR) at 1 day postoperatively (76.3% vs. 7.2%, P < 0.001) and higher stone volume clearance rates (98.11% vs. 91.78%, P < 0.001). The f-UAS group also had lower total complication rates (9.9% vs. 22.4%, P = 0.003), lower incidence of fever (5.9% vs. 11.9%, P = 0.001), shorter operative times (56.5 min vs. 59.9 min, P = 0.047), and lower usage rates of baskets (17.1% vs. 100%, P < 0.001). However, there was no significant difference in SFR at 1 month postoperatively (P = 0.627) and in the length of postoperative hospital stay between the two groups (P = 0.225). The study concludes that the f-UAS offers several advantages over the traditional UAS, including higher SFR, shorter operative times, lower incidence of complications, and reduced use of baskets.