Abstract: In this paper, the unsteady cavitating flow in a longitudinal gravity force field was researched by using numerical simulation method during vertical launch submarine missile rising. A dynamic mesh technique is adopted, combined with considering transport RANS equations of liquid mass fractions, to solve the problem of missile motion coupled with the flow field of mixture medium. The numerical results are perfectly consistent to the experimental data, thus the efficiency of the numerical method is proved. Based on the efforts mentioned above, the underwater motion process of a submarine vertically launched missile was simulated, and the relationship between the drag coefficient of the missile and cavity number was given. At last the analysis for the effect of the partial cavity on the hydrodynamics and water exiting speed was achieved.