Abstract: To understand the compositional differences between slag and slag-entrained inclusions in steel, it is important to study the evolution mechanism of slag-entrained inclusions in molten steel. The evolution mechanisms of various slag-entrained inclusions have rarely been studied through laboratory experiments. In this study, the instantaneous evolution of slag-entrained CaO–Al₂O₃–MgO–SiO₂ inclusions in steels was investigated through a series of in situ observation experiments. Large CaO–Al₂O₃–MgO–SiO₂ inclusions were designed based on the compositions of various types of industrial slag, including refining slag, tundish flux, and mold flux. To simulate the reaction process of slag-entrained inclusions in molten steel, an inclusion particle was placed on a steel surface and observed for a constant time after the steel melted using confocal laser scanning microscopy. The results showed that the CaO content decreased in the inclusions entrained from the refining slag and tundish flux. The CaO and SiO₂ contents in the - inclusions entrained from the mold flux gradually decreased over time. Additionally, the impact of the dissolved aluminum content on the compositional evolution of the CaO–Al₂O₃-based slag-entrained inclusions was investigated. Thermodynamic and kinetic analyses were employed to explore the reaction mechanism and transfer behavior during the compositional evolution of CaO–Al₂O₃-based slag-entrained inclusions. Thermodynamic analyses revealed that a higher dissolved aluminum content promoted the desulfurization reaction, which consumed CaO to form CaS in the inclusions. Kinetic analyses indicated that the reaction where the dissolved aluminum reduced the CaO was predominantly controlled by the internal diffusion of CaO and Al₂O₃ into the inclusions. Based on a diffusion-controlled kinetic model, the mass transfer coefficient for the diffusion of CaO and Al₂O₃ into the CaO–Al₂O₃-based inclusions was determined from experimental data, with a calculated value of 3.23 × 10⁻⁶ m·s⁻¹. More importantly, the current experimental method can be widely used to simulate the evolution of inclusions in molten steel from refining slag, tundish slag, and mold fluxes.