Rocket engine has the advantages of high-power density and large thrust, which is often used as the thruster to attack high speed underwater weapons. However, the outlet pressure of the nozzle pulsates violently when the rocket engine is working underwater, which affects the thrust performance of the engine and even causes accidents. To investigate the operating characteristics and wake field characteristics of underwater rocket engine, based on the VOF multiphase flow model and the ideal gas model, a numerical model of supersonic gas jet under high temperature and pressure was established. The internal and external flow field of the rocket engine was simulated under the conditions of single-phase water flow and ventilated supercavitation, and the influence of factors such as ventilated supercavity and operating pressure on the gas jet flow of the rocket engine was obtained. Results show that under the condition of fully wetted vehicle, unsteady phenomena including neck shrinkage, bulge, and back attack occurred in the gas jet flow, and the engine thrust pulsated violently. Under the condition of ventilated supercavitation, the gas of the rocket engine mixed with the gas in the supercavity and discharged, the unsteady characteristic of the wake field was significantly reduced, and there was no violent pulsation. When the operating pressure was increased to the twice of the design pressure, under the condition of fully wetted vehicle, the oscillations of gas mass flow rate and engine thrust were 30.4% and 20.6% respectively. However, the operating performance of the engine was hardly affected by the operating pressure under the condition of ventilated supercavitation.