Magnesium matrix composites exhibit outstanding properties, yet their fabrication processes remain imperfect. To address this, the interfacial characteristics, microstructures, and mechanical properties of graphene oxide (GO)-reinforced ZM5 magnesium matrix composites were systematically investigated through a combined molecular simulation and experimental approach. The calculations indicate that the bond energy of the GO/PVA-Al interface significantly exceeds that of the untreated GO/Al interfaces following PVA surface treatment. This demonstrates that surface-treated reinforcements effectively enhance interfacial bonding, thereby improving the overall composite performance. Microstructural analysis reveals that after adding 0.3 wt.% GO and undergoing squeeze-casting, the α-Mg matrix transforms from dendritic to equiaxed grains, with significant grain refinement. The precipitation of the β-Mg17Al12 second phase decreases and becomes more uniformly distributed. Compared with the matrix alloy, the GO/ZM5 composite exhibits a 23.1% increase in hardness and an 8.1% improvement in tensile strength, with the fracture morphology shifting from brittle to ductile fracture. This study demonstrates that the addition of GO significantly enhances the comprehensive mechanical properties of the ZM5 magnesium matrix composites by refining the grains and improving interfacial bonding.