Abstract: The microstructure of Al-Si coating on steel plate was observed by scanning electron microscope. The variation rule of alloy layer thickness in Al-Si coating at different heating temperature and holding time was studied by thermal simulator. The diffusion rule of iron element into the alloy layer was measured by glow spectrometer. The results show that the microstructure of alloy layer of coating is composed of Fe₂Al₅ intermetallic compound and Fe₂SiAl₇ ternary intermetallic compound before heat treatment. The microstructure of alloy layer of coating is composed of Fe₂SiAl₂, Fe₂Si₂Al₅ and other intermetallic compounds after heat treatment,and dense Al₂O₃ film is formed on the surface of coating. Silicon is enriched at the interface of alloy layer and coating layer, which inhibits the diffusion of iron element from the steel substrate to the surface of coating, and can effectively prevent the further increase of the alloy layer thickness. With the increase of heating temperature and holding time, the amount of iron element dissolved in the coating increases, and the mass fraction of iron on the surface of coating reaches 45%-55%, resulting in the thickness of alloy layer gradually increase.