The cavity, formed at the nose part of underwater launched vehicle due to the high launching velocity, will collapse while crossing water free surface and has a great impact on the strength of vehicle structure. In order to study the effect of the cavity collapse to vehicle structure, water exit dynamic analysis has been conducted. Before that, the computational model has been established. The external load, caused by cavity collapse, is assumed to be concentrated pulse forces applied on the vehicle at different time. And the flight vehicle structure is simplified into a free-free Timoshenko beam. The Fluid Structure Interaction (FSI) effect is treated as added mass and hydrodynamic drag force varying with wet surface area, which decreases with the water exit of vehicle. The dynamic response, excited by the external load, is calculated. Result presents the necessary of taking account of FSI and the influence of time space between the two concentrated forces.