This study investigated the interfacial phase evolution between magnesia refractory and low-density molten steel with different Al contents (0.5% and 10%) during the steel-slag reaction. Simulated experiments involving a refractory-steel-slag coupling reaction were conducted under laboratory conditions. The thickness and compositional distribution of the interface layers formed after the refractories were exposed to molten steel and slag were analyzed. The results indicate that, in the absence of slag, a MgAl2O4 layer is formed at the interface between the refractory and molten steel due to the reaction between [Al] and Al2O3 inclusions in the molten steel with the refractory. Upon the addition of slag, CaO in the slag is reduced to Ca and transferred into the molten steel, modifying the solid MgAl2O4 layer at the interface into a primarily liquid CaO·MgO·Al2O3 layer. The liquid interface intensified the erosion of the refractory by the molten steel, resulting in a thickened interface layer. The higher the Al content in the steel, the greater the degree of modification of the CaO·MgO·Al2O3 layer by Ca. The Mg/Ca mass transfer process during the steel-slag reaction was simulated using the FactSage Macros calculation, and the differences in mass transfer between low and high Al contents were compared. The results indicate that under high Al conditions, the supply of Mg/Ca increases significantly. The simulation results were compared with numerous findings in the literature, confirming the accuracy of the simulation.