Al/Mg/Al composite plate with thinwalled structure has the characteristics of light weight and corrosion resistance, which is an effective means for realizing light weight by utilizing magnesium alloy. However, the cracking failure at Al/Mg interface is very likely to occur during the forming process. Therefore, 5052/AZ31B/5052 threelayer composite sheet was prepared by direct hot pressing method, and their microstructure and mechanical properties were studied by combining optical microscope, scanning electron microscope, energy spectrum analyzer and universal tensile testing machine. The experimental results show that the composite sheet with good interface can be prepared at 400 ℃, 20 MPa and 2 h, and the tensile strength and elongation are 233.0 MPa and 14.0 % respectively. Furthermore, the diffusion layers with different thicknesses of Mg17Al12 and Mg2Al3 phases are formed at the Mg/Al interface, and the diffusion layer near the 5052 aluminum alloy sheet peels from the base metal during the tensile test. In addition, the average grain size of AZ31B magnesium alloy in the composite sheet increases from 4.02 μm to 5.57 μm. Thus, the above study can provide theoretical and technological foundation for the integrated preparation and forming technology of 5052/AZ31B/5052 threelayer composite sheet.