Abstract: With the optimum welding parameters, 1mm DP600 galvanized steel plateand 3mm AZ31 magnesium plate were welded by non-keyhole friction stir spot welding and transverse, longitudinal and level cutting sample were prepared, respectively, then microstructure and fracture morphology of joint were analyzed with SEM. The results show that magnesium and steel are mixed mainly in the pin stirring zone, and the "mechanical interlock" microstructure forms to increase the effective contact of the two materials to form composite enhancement.The interface of magnesium and steel at shoulder active region is smooth. In shoulder active region, intermetallic compounds and oxide form, which reduces the ductility and toughness of joint? Tensile tests of joint show the joints break away from the lap interface and fracture is ductile fracture with "vein pattern".