Abstract: To address the challenge of evaluating water impact resistance of riveted stiffened panel structures for aircraft ditching and water-air cross-medium maneuvers, this study proposes a multiscale analysis methodology based on the FEM-SPH coupled algorithm. This approach systematically investigates the fluid-structure interaction (FSI) response and failure mechanisms of riveted stiffened panels during water entry. By establishing a global FSI model simulating impact velocities ranging from 5 m/s to 30 m/s (in 5 m/s increments) and incorporating localized refined verification analyses, a multiscale characterization bridging macroscopic dynamic responses and local failure mechanisms is achieved. Results indicate that the skin water impact response exhibits a four-phase dynamic evolution pattern: Initial shock wave-dominated phase, progressive buckling development phase, hydroelastic rebound phase, and boundary oscillation attenuation phase. Orthogonal stiffeners form a synergistic load-bearing system where their stress redistribution mechanism demonstrates significant velocity dependence. Riveted joints exhibit stress gradient effects and shear force sensitivity; However, material stiffness differences (Al-7075 vs. 10 steel) in rivets show negligible influence on failure modes. Increasing skin thickness yields no significant change in peak water impact force, yet modifies the oscillation pattern of the impact load while enhancing the structural resistance to impact loading. The cross-scale analysis framework established in this research effectively addresses the limitations of global models in characterizing local failures, providing theoretical foundations for aircraft crashworthiness design and impact resistance optimization.