Abstract: 2.0 mm thick AZ31B magnesium alloy and 1.0 mm thick SPHC galvanized steel were welded by kDWJ-17 three-phase secondary rectification resistance welding machine. The fracture feature of the joint of magnesium alloy and galvanized steel by resistance spot welding was studied by the optical microscopy observation, scanning electron microscopy (SEM) and atomic binding energy. The results show that: joint fracture presented in the form of junction surface fracture and button fracture. Tensile shear of button fracture was better than junction surface rupture. Button fracture was main of ductile fracture surface, mixed little brittle fracture surface. Button fracture occured when Fe-Al compound is mainly distributed in the nugget. Grain growed coarse near the fusion line and Fe-Al binding energy is strong made the fracture location were in the fusion line. Joint surface fracture occured when Mg-Zn compound was main in the nugget. Binding energy of Mg-Zn compound is weak, which were easily pulled off.