Abstract: Resistance spot welding joint test of AZ31B magnesium alloy plate and HC260YD galvanized steel was carried out by adding pure Al powders as an interlayer via coating. The effects of Al interlayer on the microstructure and mechanical properties of joint were studied. The results show that the Mg/steel spot welded joint with Al interlayer exhibits fusion-brazing joint characteristics and is well formed. The weld consists of an nugget zone on the Mg side and Mg/steel intermetallic compound (IMC) layer. The nugget zone is predominantly constituted by α-Mg solid solution, whereas the interface zone is comprised of Fe-Al intermetallic compound (IMC) layer, which is the weak area of the joint. After adding Al intermediate layer, a fusion zone is formed near the magnesium side, and Fe-Al compound layer is formed near the steel side interface, increasing the thickness of the interface layer. The average micro-hardness of the interface area of joint with Al intermediate layer is 90.30 HV, and the tensile shear force of the joint is 4130 N. Compared with the Mg/steel spot welded joint without Al, the micro-hardness and tensile shear force have increased by 25.6% and 7%, respectively. This is mainly due to the fact that Al powder promotes the growth of Fe-Al intermetallic compounds, and the appropriate thickness of IMC layer will improve the mechanical properties of Mg/steel joints.