Abstract: To enhance the fragment-impact protection of glass fiber reinforced polymer (GFRP) casings, this study investigated the ballistic resistance of composite armor panels with carbon fiber reinforced polymer (CFRP) and poly-p-phenylene benzobisoxazole (PBO) as liner materials. Using gas gun ballistic impact tests, the ballistic performance of individual GFRP, CFRP, PBO, and their composite structures was evaluated. Numerical simulations were carried out to comparatively analyze the ballistic limit, damage modes, and energy absorption mechanisms of different panels, revealing the influence of liner material, thickness, and fragment velocity on the composite structures’ protective performance. The results indicate that, for thin liners under low fragment velocities (450 m/s), GFRP/PBO composites exhibited superior fragment-impact resistance compared to GFRP/CFRP composites. As liner thickness and fragment velocity increased, the damage mode transitioned from fiber bending and delamination on the panel’s back side to localized shear plugging at impact surface, causing the protective performance of GFRP/PBO and GFRP/CFRP composites to converge. Furthermore, due to its low density and high stiffness, CFRP proved more effective in improving the anti-penetration capability of the casing structure under high-velocity fragment impacts.