Abstract: AZ31B magnesium alloys and Q235 bare steel are joined by cold metal transfer process with AZ61 Mg based filler wire. The effects of processing parameters on weld appearance and mechanical properties are studied. The joining mechanism is studied by analyzing the microstructure and distribution of elements of welded joints. The results indicate that the tensile strength of the joints first increased and then decreased with the increase of the wire feed speed. The joint with larger weld width is fractured in the magnesium heat affected zone and has a tensile load more than 6 kN. Mg and bare steel can be efficiently joined by the thin interfacial reaction layers with Fe-Al phase, which is attributed to the diffusion of the element Al.