Abstract: Currently, the Al₂O₃ content in the high-alumina slag systems within blast furnaces is generally limited to 16wt%–18.5wt%, making it challenging to overcome this limitation. Unlike most studies that concentrated on managing the MgO/Al₂O₃ ratio or basicity, this paper explored the effect of equimolar substitution of MgO for CaO on the viscosity and structure of a high-alumina CaO–MgO–Al₂O₃–SiO₂ slag system, providing theoretical guidance and data to facilitate the application of high-alumina ores. The results revealed that the viscosity first decreased and then increased with higher MgO substitution, reaching a minimum at 15mol% MgO concentration. Fourier transform infrared spectroscopy (FTIR) results found that the depths of the troughs representing SiO₄ tetrahedra, AlO₄ tetrahedra, and Si–O–Al bending became progressively deeper with increased MgO substitution. Deconvolution of the Raman spectra showed that the average number of bridging oxygens per Si atom and the X_\textQ^3\text/X_\textQ^2 ( X_\textQ^i is the molar fraction of Qⁱ unit, and i is the number of bridging oxygens in a SiO₄ tetrahedral unit) ratio increased from 2.30 and 1.02 to 2.52 and 2.14, respectively, indicating a progressive polymerization of the silicate structure. X-ray photoelectron spectroscopy (XPS) results highlighted that non-bridging oxygen content decreased from 77.97mol% to 63.41mol% with increasing MgO concentration, whereas bridging oxygen and free oxygen contents increased. Structural analysis demonstrated a gradual increase in the polymerization degree of the tetrahedral structure with the increase in MgO substitution. However, bond strength is another important factor affecting the slag viscosity. The occurrence of a viscosity minimum can be attributed to the complex evolution of bond strengths of non-bridging oxygens generated during depolymerization of the SiO₄ and AlO₄ tetrahedral structures by CaO and MgO.