Abstract: To investigate the effect of CaO-SiO₂-Al₂O₃-MgO-CaF₂ refining slag on the composition of non-metallic inclusions, experimental and theoretical analyses were conducted to clarify the influence of Al₂O₃ content in this refining slag system on the composition of non-metallic inclusions in U71Mn heavy rail steel. The thermodynamic calculation software FactSage was used to determine that the binary basicity is 1.8, and the optimal mass fractions of MgO and CaF₂ are 7% and 10%, respectively, for the optimal desulfurization performance of this refining slag system, On this basis, refining slags with mass fractions of Al₂O₃ of 3%, 10%, 15% and 20% were prepared using a silicon-molybdenum resistance furnace and a muffle furnace. Subsequently, slag-steel reaction experiments were performed at 1 873 K using U71Mn heavy rail steel samples from a domestic steel mill. After the experiments, a scanning electron microscope (SEM) and an automatic inclusion scanning system were used to analyze the type, composition and size of non-metallic inclusions. The results show that without adding refining slag, the inclusions in the steel are mainly MnO-SiO₂-Al₂O₃ type. After the addition of refining slag, the inclusion types transformed into two categories, namely Al₂O₃-SiO₂-MnO and Al₂O₃-SiO₂-CaO. As the mass fraction of Al₂O₃ in the refining slag increases from 3% to 20%, the mass fraction of Al₂O₃ in the inclusions rises from 70% to 93%, while the mass fraction of SiO₂ decreases from 25% to 4%, and the contents of CaO and MnO remain basically stable. To explain the mechanism by which the increase in Al₂O₃ content in the refining slag leads to the rise in Al₂O₃ content in the inclusions, this study proposes a coupled interaction mechanism of the three-phase system involving refining slag, molten steel and inclusions. Theoretical analyses show that the mass fraction of Al₂O₃ in the inclusions is positively correlated with the Al₂O₃ and SiO₂ activity ratio in the refining slag, and an approximately linear relationship is observed when the Al₂O₃ content in the refining slag is less than 30%. The increase in Al₂O₃ content in the refining slag enhances its own activity and reduces the activities of CaO and MgO, thereby regulating the evolution of inclusion composition.