Abstract: The entrapment of mold powder by meniscus hook-shaped solidified shells and the tearing of molten steel at the steel level in the mold are key factors inducing slag entrainment defects in ultra-low carbon continuous casting and restricting slab quality. Previous studies have mainly focused on the mechanism of steel tearings, with insufficient attention paid to slag entrainment caused by hook-shaped shells. To fill this research gap, an optimization strategy was proposed to suppress hook growth and reduce slag entrainment by increasing the break temperature of mold flux to reduce heat transfer through the slag film. Based on conventional high-viscosity slag systems for ultra-low carbon steel, using basicity, Na₂O, F, and Al₂O₃ as single variables, combined with principal component analysis and multiple regression fitting, a target mold flux with both high viscosity （0.40 Pa·s） and high break temperature （1 216 ℃） was obtained through parameter optimization. The results show that the surface tension of the target mold flux increased to 430 mN/m, while the steady-state heat flux density decreases to 1.53 mW/m². Among these factors, cuspidine crystals precipitated in the solid slag film play a critical role in suppressing heat transfer, while the liquid slag film maintains a glassy structure to ensure adequate lubrication. Through the synergistic effects of high surface tension and low heat transfer characteristics, the target mold flux not only enhances resistance to molten steel tearing but also effectively reduces the tendency of hook-shaped solidified shells to entrap liquid slag by inhibiting the growth of the initial meniscus shell. Industrial trials further validated the effectiveness of the target mold flux. After its application, the depth of the hook structure significantly decreases, and the slag defect rate in cold-rolled coils is further reduced to 0.53%. The research results provide a feasible new way for the design of slag entrapment prevention in ultra-low carbon steel continuous casting mold flux.