The Zn-6%Al-3%Mg coated steel plates were subjected to post-heat treatments at 500 ℃-10 min and 700 ℃-10 min, respectively. Then the microstructure, phase composition and corrosion resistance of the Zn-6%Al-3%Mg coated steel plates before and after heat treatment were comparatively characterized by SEM, XRD and neutral salt spray test. The results show that the morphology, phase composition and corrosion products of Zn-6%Al-3%Mg coated steel plate after heat treatment at 500 ℃-10 min are similar to those without heat treated ones, their corrosion mass changes are basically the same. However after heat treatment at 700 ℃-10 min, the coating structure changes from single-layer of multiphase structure to multi-layer of multiphase structure, it is worth noting that there are more Fe oxides in the corrosion products, which are mainly formed by the iron-rich phase precipitated on the surface of the coating. After corrosion test, the zinc-rich surface coating on the steel substrate kept completely intact, while the corresponding corrosion mass gain is 3.2 times of the un-heat treated ones, on the other hand, the corrosion mass loss is 2.2 times of the un-heat treated ones.