Objective:To compare the effect of F7.5 ureteroscope and other types of ureteroscope (F8.5, F9.0, F9.2, F9.6) in the control of intrapelvic pressure during operation.Methods:F7.5 super thin flexible ureteroscope and other 4 kinds of disposable flexible ureteroscopes (F8.5, F9.0, F9.2, F9.6) were combined with 3 kinds of ureteral guidance sheaths to form 15 endoscope-sheath combinations. The real outer diameter of each flexible ureteroscope was measured and the effective ratio of endoscope-sheath diameter was calculated. The real outer diameter of each soft mirror was measured and the effective sheath ratio was calculated. The irrigation during lithotripsy was simulated in a 3D printed kidney model, the irrigation pressure of 50-500 cmH 2O was set, and the stable intrapelvic pressure and flow rate were measured. The line chart was drawn and the relationship between intrapelvic pressure and irrigation pressure was analyzed by linear regression. The intrapelvic pressure between different endoscope-sheath combinations and different endoscope groups were compared respectively. Results:The ratio of endoscope-sheath diameter for all combinations ranges from 0.62 to 0.92, with 0.75, 0.68 and 0.62 for the F7.5 flexible ureteroscopes and F10/12, F11/13 and F12/14 sheath combinations respectively. Intrapelvic pressure and flow rate showed an approximately linear relationship with irrigation pressure in all groups. The F7.5 flexible scope with the F10/12, F11/13 and F12/14 sheaths all provided intrapelvic pressures below 40 cmH 2O. The F12/14 sheath-F7.5 scope combination produced the lowest intrapelvic pressure of 1.47 cmH 2O at 50 cmH 2O irrigation pressure and the highest flow rate of 74.24 ml/min at 500 cmH 2O irrigation pressure. When combined with the F10/12 sheath, only the F7.5 flexible scope was able to maintain a safe intrapelvic pressure of only 25.90 cmH 2O throughout. Analysis of the variability of intrapelvic pressure using the combination of instruments as the grouping variable suggested a significant overall difference( P<0.05). Analysis of the variability between groups using the different flexible scope as the grouping variable showed that the F7.5 scope group had significantly lower intrapelvic pressure compared to the other four groups( P<0.05). Conclusions:The F7.5 super thin flexible scope has the best intrapelvic pressure control of the five flexible scopes due to the significantly reduced ratio of endoscope-sheath diameter for the instrument combination and the ability to fit a thinner guide sheath while maintaining safe pelvic pressure.