The high-pressure die castings of AZ91 Mg and ADC12 Al alloys of 3 mm were welded by frciton stir welding. OM、SEM/EDS、XRD and universal tensile testing machine were used to analyze the microstructure and tensile property of the joint. The main conclusins are: 1). Defect-free welded joints were obtained when ADC12 die-casting aluminum alloy was placed in advancing side with a welding speed of 40mm/min and the rotational speeds of 900-1300r/min. 2). Mg and Al alloys are mixed with each other in the nugget zone, and mechanical interlocking occurs. In the parent material, the thick and large casting dendrites are obviously refined, and the non-Mg/Al matrix phases mainly include Si, Mg2Al3, Mg17Al12 and Mg2Si. 3) The average tensile strength of the joints increased first and then decreased with the increase of rotating speed. Fracture occurred at the interface of the thermal affceted zone and the nugget zone in advancing side.