Abstract: The damage mechanisms and ballistic performance of ultrahigh molecular weight polyethylene (UHMWPE) fiber reinforced composite/metal armor plates were investigated under combined blast and fragment impact. The combined loading experiments were conducted with composite projectiles. The typical failure modes of the armor plates at varying impact velocities were systematically analyzed. On this basis, the finite element method was employed to validate the experimental results. Furthermore, the key influence of the panel structure layout on its dynamic response was further explored. The results demonstrate that, in the configuration with UHMWPE at the front, the rigid constraint imposed by the steel plate restricts the large deformation capacity of the fiber layers. However, it enhances the stress diffusion effect. Consequently, this configuration exhibits superior blast resistance. Conversely, in the configuration with UHMWPE at the back, the UHMWPE layer fully exerts its potential of viscoelastic deformation. It dissipates the fragment kinetic energy primarily through large deformation, resulting in better penetration resistance. The difference in protective efficacy between these 2 configurations originates from the distinct constraint effects from the material stacking sequence. This constraint effect directly governs the energy distribution mechanisms and the evolution of failure modes within the composite structure.