The effect of different surface treatments on the bonding strength of composite plates was investigated under the conditions of 400 °C and reduction ratio of 45%. Results show that the wire brush grinding treatment can only eliminate the oxide film on the plate surface, but it can hardly produce a hard layer on the plate surface. The bonding effect depends on the element diffusion promoted by the close contact between the metals on both sides of the interface. After anodic oxidation, there is a hard layer on the metal surface, and the hard layer broken during the rolling process forms a mechanical occlusion at the bonding interface. However, the hard layer cannot form an effective combination with the metal at the interface, and the bonding can only occur in the fresh metal bonding area at the crack of the hard layer. The acid-alkali washing treatment can completely remove the hard layer on the surface of both alloys without increasing the surface roughness of the plate, and the metal on both sides of the interface is more closely bonded during the rolling process. The optimal bonding strength can be obtained by surface treatment of acid-alkali washing for the aluminum-magnesium hot-rolled bonding.