In this study, to enhance the bonding strength of a solid-liquid cast rolled 6061 Al/AZ31B Mg bimetallic composite plate, an arc-sprayed Zn composite intermediate layer was deposited on the surface of an AZ31B magnesium alloy solid substrate. Through phase diagram analysis and first-principles calculation, it was observed that the introduction of a Zn interlayer changed the bonding-interface phase composition of the Al/Mg bimetallic composite plate, which positively improved the mechanical properties of the previously unaltered plate. Subsequently, the impact of Zn interlayer on interfacial microstructure, phase composition, and mechanical properties of Al/Mg bimetallic composite plates were experimentally studied. The results showed that Zn was gradually alloyed into the bonding layer of the composite plate with increasing covering plate temperature. Further, a few new phases, such as MgZn, MgZn2, and AlMg4Zn11, having strong deformation resistance and better toughness, were formed at the Al/Mg bonding interface, which decreased the content of brittle phases such as Al3Mg2 and Al12Mg17. In addition, it was also found that the mechanical properties improved with the increase in covering plate temperature. At a covering plate temperature of 680 °C, the tensile strength and shear strength reached their peak values of 136 MPa and 74 MPa, respectively. Specifically, the tensile and shear strengths of the Al–Mg bonding layer with Zn interlayer increased by about 25% compared with the composite Al–Mg bonding layer without the interlayer. Contrastingly, at the covering plate temperature of 700 °C, while the diffusion of elements and thickness of the binding layer increased, the tensile and shear strengths decreased to 109 MPa and 54 MPa, respectively.