Abstract: To address the composite electromagnetic scattering of large-sized targets and complex sea surfaces under high-sea-state conditions, this study proposes a hybrid algorithm that combines an improved Shooting and Bouncing Ray (SBR) method with the Weighted Curvature Approximation (WCA). In the SBR method, a K-dimensional tree (kd-tree) and reverse tracing of adjacent surface elements are introduced to improve ray-tracing efficiency, and the Physical Theory of Diffraction (PTD) is applied to correct edge diffraction. The sea surface is generated based on the Elfouhaily spectrum, with broken waves approximated using a cleaved-wave structure. Scattering from each surface element is calculated using WCA, whose kernel function incorporates a local curvature weighting factor to overcome the limitation of perturbation methods, which are only suitable for small-scale undulations. Large-scale non-Bragg scattering from the sea surface and multipath scattering from broken waves are uniformly handled by the improved SBR. Composite scattering decomposes the total field into target, sea surface, and six types of coupled paths, and multiple reflections are calculated through unified ray tracing. Simulations show that the improved SBR method increases efficiency by several times to two orders of magnitude while maintaining accuracy; the radar cross section (RCS) of a sea surface with broken waves is consistent with measured trends. In composite scattering involving large targets such as ships and a sea surface with broken waves, the target’s own scattering dominates, while the influence of broken waves on the total rearward RCS is small and can be approximately neglected.