<p>To achieve the goal of deep desulfurization of non-oriented silicon steel during secondary refining, desulfurization experiments were conducted through slag–steel reactions to investigate the effects of different compositions in the CaO saturated CaO–Al₂O₃–CaF₂–SiO₂–MgO slag on desulfurization kinetics. The mechanisms influencing the reaction were analyzed, and the rate-controlling step of the process was systematically studied. The results indicate that the CaO–Al₂O₃–SiO₂ slag with an addition of CaF₂ achieves a desulfurization efficiency of 93% within 10 min, reducing the sulfur content in steel to 0.0015% ± 0.0002%. The equilibrium sulfur distribution ratio and overall sulfur mass transfer coefficient both increase with rising CaO/(Al₂O₃ + SiO₂) and optical basicity in the liquid slag, indicating consistent trends between them. Compared with the CaO–Al₂O₃–SiO₂ slag, the appropriate addition of CaF₂ increases the liquid phase fraction of the slag, enhances slag fluidity and mass transfer efficiency, and significantly improves the mass transfer coefficient of sulfur in slag as well as desulfurization efficiency. A comprehensive analysis of the effects of liquid phase fraction on viscosity and mass transfer indicates that the overall mass transfer coefficient reaches its peak when the liquid phase fraction ranges from 0.53 to 0.75. As the thermodynamic conditions are optimized, the overall mass transfer coefficient reaches a plateau, and it is confirmed that the plateau value of the overall mass transfer coefficient is close to the mass transfer coefficient in the metal phase. At this stage, the primary rate-controlling steps of the reaction are the mass transfer of sulfur in the metal phase.</p>