<p>The influence of Al₂O₃ on the viscosity, desulfurization, and alkali removal properties of CaO–SiO₂–MgO–Al₂O₃–Na₂SiO₃ slag was explored based on the high-temperature experiments, X-ray photoelectron spectroscopy analysis (XPS), and thermodynamic calculation. With an increase in Al₂O₃ content, the viscosity tended to increase, but the molten slag still had the satisfactory fluidity at 1500 °C. The viscosity of the slag was 0.36 Pa·s when Al₂O₃ content was as much as 17.5wt.%. Al₂O₃ promoted the generation of more Al–O bonds and [AlO₄]-tetrahedron. The amount of bridging oxygen (O⁰) increased, while that of non-bridging oxygen (O⁻) and free oxygen (O²⁻) decreased, consistent with the changes in the corrected optical basicity. The sulfur partition ratio (<i>L</i>s) and sulfur capacity (<i>C</i>_S) of the slag decreased because the overall basicity of the slag decreased and more Ca²⁺ was occupied for charge compensation of [AlO₄]. As the Al₂O₃ content increased, the alkali removal rate and alkali capacity of the slag increased because of the decrease of O²⁻ content and the increase of Na⁺ stability, which inhibited the reaction between Na⁺ and carbon. In conclusion, the viscosity and desulfurization performance of high-Al₂O₃ slag met the smelting requirements of BF, but the alkali capacity of the slag was improved, which was conducive to the application of alkali-bearing raw materials from this perspective.</p> Graphical Abstract <p></p>