Laser welding of DP600 dual-phase steel to AZ31 deformation magnesium alloys is carried out in steelon- magnesium overlap configuration by using fiber laser as heat welding source with the addition of Sn foil. The best welding has been achieved based on the optimization of welding parameters, and microstructure, phase distribution, grain size, elements distribution and phase compositions of the welding joint are studied by using the horizontal microscope, scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and electron backscatter diffraction (EBSD) probe and X-ray diffraction (XRD). The results show that joining iron to magnesium can be realized due to the formation of new phases in welding transition region, and it is suitable for laser welding steel to magnesium alloys with the addition of Sn foil. In addition, the experimental results suggest that there are no softening organization in heat affected zone of the upper dual-phase steel, some defects, such as oxides, pores, etc can be avoided in the transition region of steel/magnesium. Besides, FeSn, Fe1.3Sn and Fe3Sn phases are formed in the transition zone of steel side, while columnar Mg2Sn phase is seen in the transition region of magnesium side, which is befit for jointing steel/Mg dissimilar metals by the metal lurgical reaction among the iron, Sn and magnesium.