In this paper, a study of the high-speed gas jet of a rocket nozzle underwater was carried out using commercially available CFD software FLUENT with it’s user-defined-function.The volume of fluid technique based on finite volume method was adopted to solve the time-dependent multiphase flow including a compressible phase, and the PISO algorithm was included.The computed results show that this problem was calculated successfully.The gas bubble behind the nozzle, and the wave structure existing in highly compressed gas in water were captured accurately.