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For coprecipitating vehicle, water ramjet is an ideal way to reduce drag. In this paper, the performance of water ramjet at large depth and the flow field structure at nozzle are simulated and analyzed. Through the numerical simulation method of underwater Jet flow, this paper studies the development process of flow field at the initial stage and the parameters variation with time of Jet flow in steady state when the working depth is 200meters. The results show that at the beginning of the jet flow, the water blocking effect seriously prevents the development of the gas bubble, and the gas bubble turns over at the nozzle outlet, and expands in length and width. The unsteady process of ‘necking expansion necking’ occurs repeatedly at the nozzle outlet, with the pressure, velocity and temperature change dramatically near the nozzle outlet. When the flow field is fully developed, the pressure, density, flow rate and thrust of the nozzle outlet fluctuate repeatedly with time, and finally tend to a stable value after many oscillations.