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To investigate the complex multiphase flow of supersonic gas jets in still water, an underwater ignition experiment of solid rocket motor is conducted in the pressure tank, and the numerically simulated results of gas-water coupling for the same case are obtained by using Reynolds-averaged Navier-Stokes(RANS) method and volume of fluid(VOF) model. The results show that, after underwater gas jets achieve supersonic flow, the gas bubble appears to be cap-like due to the jet impact effect. Then the gas bubble gradually evolves into an ellipsoid-shaped gasbag with average axial expansion velocity of about 40 m/s. The flow structures inside the gas bubble are very complex, where two shear vortex rings and repeated shock cells exist. The interaction between the boundaries of gas jet and gas bubble may induce the instability of gas jet in the subsequent evolution stage. It is also demonstrated that there exists a part of high pressure in the flow field, and the pressure peaks gradually match with the environmental pressure in the oscillation process. The pressure of opening nozzle closure and the area of nozzle outlet are the two important factors that affects thrust peak value. [ABSTRACT FROM AUTHOR]