The effects of Si and SiO2 content in the embedding powder on the equilibrium partial pressure of halide gas were compared by material thermodynamic calculation, and higher SiO2 content facilitated the preparation of silicon-modified aluminide coatings. The microstructure and hot corrosion behavior of silicon-modified aluminide coating prepared by different SiO2 contents were studied. As the SiO2 content in the embedding powder increased, the Al, Si, and Cr content was elevated in the prepared coating surface, which promotes the Cr9.1Si0.9 phase precipitate in the coating. Besides, the corrosion weight loss of the coating was the lowest, after 50 wt% NaCl + 50 wt%Na2SO4 mixed molten corrosion at 900 °C for 100 h, showing the excellent corrosion resistance. At the initial corrosion stage, the higher Si content induces the formation of a SiO2 oxide film on the coating surface, which could resist the mixed molten salt reaction. As the corrosion process, the SiO2 oxide film and the below Al2O3 oxide film peel off due to thermal expansion mismatch. Meanwhile, the coating relies on the dispersed precipitation below the oxide film to hinder the corrosive elements and the consumption of Al elements at the final corrosion stage.