Abstract: The low-reactivity mold flux with low SiO₂ content is considered suitable for the continuous casting of high-aluminum steel since it can significantly reduce the reaction between Al in steel and SiO₂ in mold flux. However, the traditional low-reactivity mold flux still presents some problems such as high viscosity and strong crystallization tendency. In this study, the co-addition of Li₂O and B₂O₃ in CaO–Al₂O₃–10wt%SiO₂ based low-reactivity mold flux was proposed to improve properties of mold flux for high-aluminum steel, and the effect of Li₂O replacing B₂O₃ on properties of mold flux was investigated. The viscosity of the mold flux with 2wt% Li₂O and 6wt% B₂O₃ reached a minimum value of 0.07 Pa·s. The break temperature and melting point showed a similar trend with the viscosity. Besides, the melt structure and precipitation of the crystalline phase were studied using Raman and X-ray diffraction spectra to better understand the evolution of viscosity. It demonstrated that with increasing Li₂O content in the mold flux from 0 to 6wt%, the degree of polymerization of aluminate and the aluminosilicate network structure increased because of increasing Li⁺ released by Li₂O, indicating the added Li₂O was preferentially associated with Al³⁺ as a charge compensator. The precipitation of LiAlO₂ crystalline phase gradually increased with the replacement of B₂O₃ by Li₂O. Therefore, Li₂O content should be controlled below 2wt% to avoid LiAlO₂ precipitation, which was harmful to the continuous casting of high-aluminum steels.