Abstract.Reduction ratio is one of the most important parameters that affect final bonding strength of laminated composites fabricated by hot rolling. In general, the bonding strength increases with the reduction ratio. However, it was found that the reduction ratio had a reverse effect on the bonding strength of the 7075 Al/AZ31B Mg/7075 Al laminated composites prepared by hot rolling. Microstructure, elements distribution, and phase constitution in the bonded interface of the laminated composites at different reduction ratios were investigated, and the hot rolling process was simulated by finite element methods (FEM). The results show that a larger reduction ratio would lead to: a wider of diffusion layer with a relatively lower strength; a larger secondary tensile stress; and a larger relative slipping distance after the formation of the bonded interface. All of them induced the reverse effect.