The joining of Mg alloy to steel was realized by metal inert-gas arc welding, and the weld thermal cycle characteristics and Mg-steel joints were investigated. The results show that the temperature distribution in the joints is uneven. Mg alloy welds present a fine equiaxed grain structure. There exists a transition layer consisting mainly of AlFe, AlFe3 and Mg（Fe, Al）2O4 phases at Mg/steel interface, and it is the weakest link in Mg？steel joints. The welding heat input and weld Al content have the significant effect on the joint strength. The joint strength increases with increasing the heat input from 1680 J/cm to 2093 J/cm, due to promoting Mg/steel interface reaction. When weld Al content is increased to 6.20%, the joint strength reaches 192 MPa, 80% of Mg alloy base metal strength. It is favorable to select the suitable welding heat input and weld Al content for improving joint strength.
