Double layers composed of Sn and Mg, each 0.8 μm thick, were fabricated on a hot-dip galvanized steel (8.4 μm) sheet using DC magnetron sputtering and post-annealing processes. With an increase in temperature, the surface morphologies were agglomerated with each other. Additionally, Sn/Mg mixture sites, including an intermetallic compound of Mg2Sn, were formed at 190 °C and locally clustered at 220 °C. In the salt-spray test, the corrosion resistance of the Sn/Mg film prepared at 190 °C was 960 h, which is longer than that at non-heat for 528 h or 220 °C for 480 h. In the polarization test, the Sn/Mg film formed at 190 °C displayed a lower corrosion current density of 1.07 μA/cm2 and potential of 1.62 V/SSCE than those at non-heat or 220 °C.