Abstract: Supercavitating projectiles travel underwater at high speed and long distances through the supercavitation drag reduction technology, which is an effective means to counter close-range underwater threats. In order to expand the defense range and increase the lethality, the supercavitating projectile has a high launch speed. The high-speed supercavitating projectile is subjected to a great impact load during water entry process, and a significant structural deformation occurs on the projectile. There is an interaction between the structural deformation and the flow field. Resultantly, the regular simulation research method based on the rigid body assumption is no longer applicable. To study the structural deformation of the high-speed supercavitating projectile and its influence on the hydrodynamic characteristics, a bidirectional fluid-structurer interaction simulation model of the high-speed projectile is established by coupling the fluid dynamics solver and the structural dynamics solver. The accuracy of the numerical method to calculate the supercavitation flow field and the fluid structure interaction are validated by comparing with the published results. Numerical simulation investigations on the supercavitation flow field and the structural deformation characteristics of the high-speed projectile during water entry at different initial angles of attack are carried out with the bidirectional fluid-structure interaction method. By comparing the calculation results of the fluid-structure interaction model and the rigid body model, the influence of structural bending deformation of the supercavitating projectile on its hydrodynamic load is obtained. Research results show that the fluid-structure interaction effect has a significant influence on the supercavity and hydrodynamic load. When considering the fluid-structure interaction effect, there is positive feedback between the hydrodynamic load and the bending deformation of the supercavitating projectiles; the stress, strain and the hydrodynamic load of high-speed supercavitating projectiles increase significantly with the increase of the initial angle of attack. The structure of the projectile is safe when the initial velocity is 1400 m/s and the initial angle of attack is below 2°.