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 for welding test, and analyzed organizational structure and distribution of composition of the joints. A low-melting extrusion mechanism of compound about zinc and magnesium was proposed. The function of Zn in magnesium alloy and galvanized steel by resistance spot welding were analyzed. Zn and Mg can form low melting compounds.They can fill the gap due to welding deformation to make the reaction interface sealed. They assist the diffusion of Fe and Al at the interface. Fe and Al at the interface occurs Fe₂Al₅ compound. It forms high-strength welding joints of magnesium alloy and galvanized steel by resistance spot.