The microstructures and morphologies of Al/ZE42/Al composite plates were observed by optical microscopy (OM), scanning electron microscopy (SEM). Corrosion rates of the Al/ZE42/Al composite plates at different temperature and holding time were also measured in salt water immersion test, and the influence of annealing on interfacial microstructure of Al/ZE42/Al composite plates and corrosion behaviors in 5.0 wt% NaCl aqueous solution were investigated. The results showed that element diffusion of Mg, Al, Zn, Y and Er in Al/ZE42/Al composite plate interfacial region after annealing was occurred. The interfacial diffusion layer has two reaction layers. Mg17Al12 phase in one reaction layer near ZE42 magnesium alloy, and Al3Mg2 phase in the other reaction layer closed to Al plate. With annealing temperature and holding time increasing, the thickness of ZE42/Al interfacial diffusion layer increased, and at the meanwhile recrystallization behaviors occurred in ZE42 magnesium alloy. Annealing cannot improve corrosion resistance of Al/ZE42/Al composite plate, but improving corrosion rate. The corrosion mechanism of the Al/ZE42/Al composite plate is the aggravation of galvanic corrosion which occurred from edge to internal of the plate.