The microstructure of Mg solid solution layer of Mg/Al composite plates by multi-pass friction stir processing (FSP) was studied by using EDS and TEM. The transition zone without defects was formed at the interface of Mg/Al composite plates. According to the element distribution, the transition zone can be successively divided into five layers: Al solid solution + Al3Mg2 layers, Al3Mg2 layers, Al12Mg17 layers, Mg solid solution + Al12Mg17 layers, and Mg solid solution layers, respectively. The rectangular Mg2Si phases with a size of about 500 nm × 300 nm were observed in the Mg solid solution layer with Mg and Al mass fractions of 92.56 wt % and 3.75 wt % respectively. The crystallographic orientation relationship between Mg matrix and Mg2Si phases is [221]Mg//[001]Mg2Si, (1‾100)Mg//(2‾1‾0)Mg2SiThe simulation results show that the interface of Mg solid solution and Mg2Si phases is coherent interface that has a very high lattice matching degree. Furthermore, there are a large number of nanoparticles with a size of about 20 nm in Mg solid solution layers. The multiple micro-twins with ranging from 4 to 6 atomic layers were observed among the granular structure. The micro-twins were parallel to each other and through the whole granular structure from one side of boundary. At the same time, serious lattice distortion existed in granular structure, and there were a lot of dislocations and stacking faults.