<p>In metallurgical processes, understanding the viscosity of high-alumina slag is pivotal for enhancing the efficiency of slag–metal separation for high-temperature smelting of low-grade complex iron ore. The present study investigates the influence of binary basicity, as well as the contents of Al₂O₃ and MgO, on the flow properties, viscous activation energy, and structural attributes of high-alumina slags. This investigation employs a dual-method approach, combining FactSage thermodynamic calculations with rotating cylinder experiments. The results reveal that, at a specified temperature, an increase in slag basicity from 0.8 to 1.2 or an elevation in MgO content from 6 wt.% to 10 wt.% leads to a decrease in the viscosity of the CaO-SiO₂-MgO-Al₂O₃ slag system. This reduction enhances the fluidity of the slag. Concurrently, the viscous activation energy of the slag is diminished, and its intricate structure undergoes depolymerization as slag basicity and MgO content rise. Conversely, when the Al₂O₃ content escalates from 25 wt.% to 35 wt.%, the viscosity of the slag correspondingly increases, resulting in deteriorated flow properties. In tandem, the viscous activation energy of the slag rises, and slag polymerization is augmented.</p>