By using the commercial CFD software Fluent 6.3, the three-phase (water, air, vapour) flow field with natural cavitation was established. The UDF was used to resolve and control the missile's movement. It realized the numerical calculation of unsteady high-speed water-entry flow field, which consider natural cavitation as well as the missile's multiple degree of free movement. The Mixture was adopted as the multi-phase model; and the stand [WTBX]k-ε[WTBZ] model was used as the turbulence model; and SIMPLE and Standard were used as pressure-velocity coupling method and discretizaion method respectively. The Layering was adopted as the dynamic mesh updating method and the UDF can output the parameters real time. In this paper, the missile high-speed water-entry flow field with 20° angle was simulated and analyzed, and the influence of the thrusting force on cavity development and the missile hydrodynamics were gained.