Abstract: The magnesium alloy AZ31B sheet to DP600 galvanized steel sheet were spot welded by friction stir spot welding (FSSW) without keyhole under the condition of the optimal welding parameters. The sound joint with strength of 8. 7 kN was obtained. The whole spot welding process was divided into three phases,pin inserting (I),pin stirring and withdrawing (II), pin withdrawing finished (III). The microstructure analysis showed that in the welding nugget zone,the grain of steel became lathy that flowed significantly. Both the steel and the magnesium alloy in the thermo-mechanically affected zone underwent a dynamic recrystallization process resulting in a coarse grain. The grain of the steel was longer and the grain magnesium alloy became much more coarsened. In the stirring pin withdrawing zone,the grain size of magnesium alloy was small with a uniform distribution. These equiaxed grains were completely transformed. In the heat affected zone,the steel grain was thicker and longer, and the magnesium alloy grain was coarse. The element distribution analysis showed that in the first stage,the interface between magnesium and steel had no diffusion,while in the stage II and stage III,the diffusion of main elements occured in the nugget area.