MgO·Al2O3 spinel type inclusions in casing steel were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).The results show that there are three forms.One is pure MgO·Al2O3 spinel,another is the composite oxide of the Mg-Al-Ca-Si-O system,and the third is the complex with oxide as a core covered by sulfide.The formation mechanisms were studied.The influences of slag basicity and vacuum degree on the magnesium content during the vacuum treatment of molten steel and furnace lining in molten steel were calculated with the coexistence theory of slag structure.The results show that the magnesium content increases with the increase in slag basicity and aluminum content in molten steel,and decreases with the increase in CO partial pressure. MgO · Al2O3 spinel type inclusions in casing steel were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results show that there are three forms. One is pure MgO · Al2O3 spinel, another is the composite oxide of the Mg-Al-Ca-Si-O system, and the third is the complex with oxide as a core covered by sulfide. These influences of slag basicity and vacuum degree on the magnesium content during the vacuum treatment of molten steel and furnace lining in molten steel were calculated with the coexistence theory of slag structure. the results show that the magnesium content increases with the increase in slag basicity and aluminum content in molten steel, and decreases with the increase in CO partial pressure.