During the continuous casting of high-aluminum steel, important components of the mold flux can be easily reduced by the aluminum in the molten steel. Consequently, the mold flux performance is deteriorated, impeding the smooth running of the continuous casting and affecting the quality of the cast slab. To solve this problem, thermodynamic calculations and laboratorial crucible experiments were performed to investigate the reduction of different mold fluxes by the aluminum in molten steel. Plant trials based on the laboratory studies were performed. It was found that SiO₂, MnO, Na₂O and B₂O₃ reacted with the aluminum in molten steel, while CaO, CaF₂, MgO, Li₂O and BaO did not. Since the composition contents of in CaO-Al₂O₃-based mold flux were changed only slightly after steel/slag interfacial reactions, a stable CaO-Al₂O₃-based mold flux was tested in industry production. The application effect of this stable mold flux was very good.