Hot dip tin coating was obtained on AZ91D magnesium alloy surface, and interfacial reactions between Sn coating and electroless Ni-P plating was studied. The results show: after 220±5°C, 2h diffusion, the intermetallic compounds possessed corresponding thickness, and Ni3P layer along the interface acted as a good diffusion barrier between Sn and Ni. When Ni3Sn4 spread to the surface, microcracks were observed on the surface. Corrosion resistance of Ni(P)-Sn coating before and after diffusion was investigated with polarization curves. The results show: Ni(P)-Sn coating significantly enhance the corrosion potential of base material, but after the Ni3Sn4 growth to the surface, it caused electrochemical corrosion and decreased the corrosion resistance.