The billets of 6063 aluminum alloy with the thickness of 0.2 mm and AZ80 magnesium alloy with the thickness of 5.0 mm was selected to assembling and the thickness ratio was set as 20. The microstructure and mechanical properties of Al/Mg alloy plate with large thickness ratio were analyzed under various hot rolling reduction rates by hot rolling. The results show that when the hot rolling reduction rate reaches 45% or higher, Al/Mg alloy plate forms an interface with excellent bonding performance, and the fine grains are evenly distributed in the magnesium matrix. After increasing the hot rolling reduction rate, the grain size of matrix decreases continuously. At this time, the smaller grain size dispersion coefficient is formed, more grains are crushed, and the grain distribution is more uniform. After increasing the hot rolling reduction rate, the Al/Mg alloy plate with larger thickness ratio and higher yield strength is obtained, and the material undergoes more obvious work hardening. The tensile strength increases first and then decreases, and the maximum tensile strength is obtained when the hot rolling reduction rate reaches 55%. The number of dimples increases significantly when the hot rolling reduction rate reaches 65%, indicating that the magnesium alloy obtaines stronger toughness through dynamic recrystallization transformation. The yield stress fluctuates obviously. The highest yield strength is obtained when the hot rolling reduction rate is 35%, and the lowest yield strength is obtained when the hot rolling reduction rate is 65%. After increasing the hot rolling reduction rate gradually, the yield stress also decreases continuously.