Abstract: As to underwater launch, it is important to study the characteristic of hydrodynamic forces and structural response. It includes complex flow with phase change, structural behavior under time-vary constraints and coupling effects between fluid and structure. Loosely coupled method is utilized in the present work, in which the structural solver is linked to the flow solver. Governing equations of fluid dynamics and structural dynamics are solved respectively. The coupled algorithm is achieved by exchanging data through the interface of fluid domain and solid domain every time step. The flow model is based on RANS equations. In this model, mixture model is used to simulate multiphase flow, a cavitation model is introduced to describe the phase change, a modified turbulence model is utilized to simulate the turbulent effect of mixture, and the dynamic mesh technique is adopted to deal with the moving boundary. Rigid motion and structural vibration are calculated respectively. The structural model is established based on the equivalent beam model, and computed by the time domain integral method in Body Axes System. The fluid-structure coupling method is set up aimed at underwater launch process and has been validated by the experiment. This method could not only capture the evolution of the natural cavitation, but also obtain hydrodynamics forces, structure vibration and bending moment of the projectile. We utilize the proposed model to study the influence of structural stiffness and launch velocity on coupling effect of cavity collapse and structural vibration. The results show that structural load is mainly affected by collapse pressure including pressure amplitude, acting location and phase relationship between collapse pressure and structural vibration.