Abstract: The magnesium alloy AZ31B sheet and galvanized steel sheet were lapped by cold metal transfer spot welding. Using orthogonal test method to optimize process parameters and at the same time,by means of optical microscope,scanning electron microscopy and universal tensile testing machine,the microstructure and mechanical properties of welding joint were studied. The results show that satisfied weld appearance and properties can be obtained. The important sequence of process parameters are followed,size of hole on the galvanized steel sheet,wire feed speed,spot welding time. The joints are typical spot welding-brazing joint,which are composed by the brazing zone and the welding zone. The tensile shear load of joint can reach 3.12 kN,which is larger than that of Mg-Mg spot welding joints. The shear fracture and tear fracture dominate the fracture mode of the joint.