Interface plays a crucial role in the mechanical properties of composite materials. In this study, the interfacial morphology, microstructure and deformation mechanism of an explosively welded AA6061/AZ31B composite plate were both experimentally and numerically investigated. SEM analysis showed that the bonding interface has a periodic wavy shape with melted (vortex) zones. The vortex formation was found directly related with the deformation of the base plate. The compound composition found in the vortex was caused by the jetting at the interface. Owing to the different crystal structures and thermal/physical properties of the AA6061 and AZ31B, deformed grains were dominant at the Al matrix side near the bonding interface, while the Mg matrix side contained mainly recrystallized grains. Mg17Al12 intermetallic compounds were found in the melted zones through EDS and EBSD analyses. TEM analysis showed that a 2.5-m-thick transitional layer of Mg17Al12 phase was formed at the bonding interface. Nanoindentation test results showed the melted zone had a high nano-hardness, owning to the formation of a hard and brittle Al-Mg intermetallic compound there.