Hot stamping is an alternative technology to produce ultra-high strength steel (UHSS) with a tensile strength above 1 GPa for automotive bodies. At present, the hot-dip Al-10%Si (mass fraction) coating is used as a shield coating for the hot stamping steels, which protects the steels from surface oxidation and decarburization, and enhances their corrosion resistance. However, the microstructure evolution and compounds of hot-dip Al-10%Si coating during austenitization of 22MnB5 hot stamping steel are not clear yet. In this work, the thermo-mechanically induced microstructure evolution of hot-dip Al-10%Si coating is observed using SEM after austenitization of 22MnB5 hot stamping steel at 900 ℃ for different times, and the elemental depth profiles are analyzed in hot-dip Al-10%Si coating by EDS and GD-OES. The results show that before austenitization, the hot-dip Al-10%Si coating consisted of an aluminum matrix, pure silicon, and the intermetallic compound Fe₂SiAl₇, which was formed by eutectic reaction, there was a thin layer, which was composed of Fe₂Al₅ and FeAl₃ between the intermetallic compound Fe₂SiAl₇ and the steel substrate. When 22MnB5 hot stamping steel was austenitized at 900 ℃, the ternary eutectic phase Al+Si+τ₆ was transformed into an Al-Fe-Si ternary intermetallic compound or Fe-Al binary intermetallic compound gradually in the hot-dip Al-10%Si coating. When the austenitizing time was 2 min, the Al-10%Si coating was composed of the intermetallic compound Fe₂SiAl₇, Fe₂Al₅ and FeAl₂ phases; when the austenitizing time was 5 min, the Al-10%Si coating was composed of FeAl₂, Fe₂SiAl₂ and Fe₅SiAl₄ phases; when the austenitizing time was 8 min, the Al-10%Si coating was composed of FeAl₂ and Fe₅SiAl₄ phases. Because the diffusion rate of Al atoms was much larger than that of Fe atoms in the diffusion layer of intermetallic compound Fe₂SiAl₂ and coating/steel substrate, the amount of Al atoms which diffused and reacted from the coating to the grain boundaries or grain of steel substrate was much larger than that of the Fe atoms which diffused from the steel substrate to the Al-10%Si coating, also the number of vacancies which diffused from the steel substrate to the Al-10%Si coating was much larger than the other way round. Due to this imbalance, the Kirkendall void was formed in the interface between the diffusion reaction layer and the Al-10%Si the coating. The hot-dip Al-10%Si coating can be used as the protective layer, since it has a stable Al₂O₃ film formed on its surface, and its thermal oxidation was very limited, during the 22MnB5 hot stamping steel austenitizing. But the protective performances of Al-10%Si coating could be poor, because the high temperature ductility of brittle intermetallic compound was low, which induced a lot of micro cracks that were perpendicular to the interface of coating/steel substrate, and penetrated the whole coating during the diffusion process of hot-dip Al-10%Si coating.