In the field of aerospace, the structural integrity analysis of the revolution body, such as those dominated by missiles and rockets, is becoming increasingly important. However, addressing the water entry problem in water-gas two-phase structures presents a complex fluid-structure interaction challenge. The intricate shapes and internal configurations of these structures further complicate the analysis of water and structural loads. In this paper, a multi-stage revolution model comprising internal and external compartments is established, based on the SPH-FEM (coupling Smoothed Particle Hydrodynamics method and the Finite Element Method) numerical calculation method. The study examines water inflow scenarios for revolution structures under three different working conditions. Various results, including velocity, overload, pressure, cross-sectional force, and stress, are analyzed. The comparison results indicate that the maximum bending moment in the multi-stage revolution body typically occurs near the support connection between the internal and external compartments. As the water entry height and pitching angular velocity increase, the water load grows larger, the center of action shifts backward, and the maximum bending moment in the critical cross-section rises, leading to potential collisions between the internal and external compartments.