Abstract: Aiming at the complex multiphase flow characteristics during the underwater ignition of solid rocket motors(SRM), a gas jet calculation model based on the VOF model was established to conduct simulation investigations on gas jet behavior at different ignition water depths. The evolutionary characteristics and differences of the flow field structure were analyzed, and the influence of water depth on the motor thrust and wall pressure oscillation was further revealed. The results show that the gas jet evolution of the gas jet exhibits distinct three-stage features: the initial stage of flow field evolution, the oscillation development stage, and the fully developed stage; with the increase of ignition water depth, the ambient back pressure rises, causing the working state of the nozzle to change from under-expansion to over-expansion; the increase in over-expansion degree leads to the intense water compression of the gas jet, reducing the maximum radial size of the bubbles by 43.2%. Correspondingly, the gas oscillation transforms into the expansion-narrowing-breakage process. At the same time, the reciprocating motion of the shock wave aggravates the instability of the gas jet, causing the thrust pulsation amplitude to increase and the average thrust coefficient of the motor to decrease by 16.5%; the pressure oscillation of the nozzle outer wall surface is highly consistent with the thrust fluctuation frequency, indicating that the unsteady wall force on the outer wall surface is the dominant mechanism inducing the thrust pulsation.