Abstract: Three-phase secondary rectification spot welding machine was used to conduct dissimilar metal joining of magnesium alloy to steel,and the window of welding parameters and optimum parameters were obtained.The microstructure of the joints was examined with optical microscope and the microhardness in different zones was measured.The results show that the shearing strength increased initially and then decreased with the increase of welding time T (2-14 cycle),welding current I (20-37.5 kA) and electrode pressure P (5-8 kN).The maximum shearing strength reached 6.961 kN when T was 8 cycle, I was 32 kA,and P was 7 kN.The joint was composed of nugget on magnesium alloy side,with equiaxed grains and columnar grains,and heat-affected zone on steel side,with lath martensite.On magnesium side,the closer distance of measured point to the fusion line,the higher the hardness was.The highest hardness appeared in the center of steel sheet on steel side.