Protecting spacecraft against hypervelocity impacts (HVIs) of space debris, which may cause fatal damage to the spacecraft, has attracted much attention. In this study, we conduct the numerical simulation of HVIs of a projectile on rear walls protected by a debris shield. To numerically simulate the HVIs, an improved smoothed particle hydrodynamics (SPH) method with new particle generation and particle merger techniques is used. Through the numerical simulation, we evaluate the total kinetic energy of debris cloud through unit area on the top surface of rear wall, and discuss the ballistic limits of the graphite/epoxy (Gr/Ep) and hybrid composite rear walls. Consequently, we provide the data needed for designing the rear walls.