Lightweight refractory linings for industrial furnaces have become important subjects of development in high-temperature industries. The reaction mechanism between a lightweight microporous alumina-based refractory material and molten steel was investigated in this study. The main mechanism of refractory damage was structural spalling, caused by steel penetrating the pores. The many micropores in lightweight microporous alumina have high specific surface area and reactivity, inducing the formation of FeO-Fe2O3-Al2O3 phases. This impeded the further penetration of molten steel and the direct dissolution of refractory oxides, promoting greater resistance to molten steel than that shown by common tabular alumina-based refractories, in which Fe does not react and steel penetration through the pores cannot be retarded.