To clarify the evolution mechanism of the gas curtain flow field during underwater launch and optimize the in-tube drainage efficiency， this study investigates the effects of different injection pressure conditions on the evolution characteristics of the gas curtain inside a spiral-grooved underwater gun tube. A transient three-dimensional two-phase flow model was established for the drainage process during underwater gas-curtain launch. Based on a 40 mm supercavitating projectile and a spiral-grooved gas-curtain launch tube， numerical simulations of gas curtain evolution were performed under four different injection pressure conditions， and the effect of the pressurization rate on key evolution characteristics was analyzed. The results show that a higher pressurization rate results in better overall drainage performance， but it also induces higher pressure ahead of the projectile. Specifically， as the pressurization rate increases from 1 MPa·ms^-1 to 4 MPa·ms^-1， the drainage completion time （i.e.， the time required for the gas curtain front to reach the muzzle） decreases from 14.4 ms to 11.8 ms， achieving an 18.1% improvement in time efficiency and a 25.7% increase in drainage capacity. However， upon completion of drainage， the pressure on the projectile surface increases from 3.7 MPa to 4.96 MPa， and the average pressure inside the tube rises from 4.14 MPa to 5.47 MPa. In conclusion， although a high pressurization rate can significantly accelerate drainage， it substantially increases the subsequent in-tube motion resistance of the projectile. Therefore， in the practical matching design of propellant charge and projectile， it is necessary to comprehensively balance drainage efficiency and initial motion resistance， and reasonably control the injection pressure gradient.