Abstract: The high-temperature properties of the Al₂O₃–CaO–SiO₂–Fe₂O₃ basic slag had significant influences on steelmaking operations and waste slag utilization. To further clarify the structural characteristics and properties of Al₂O₃–CaO–SiO₂–Fe₂O₃ slags, the structures and viscosities of the slags were researched. The slag liquidus temperature was determined, which decreased from 1365 to 1287°C after 4.16wt%–8.52wt% Al₂O₃ was added to the slags and then increased to 1356°C after 17.07wt% Al₂O₃ was added. Structure analysis indicated that increasing temperature depolymerized the structure of the 4.16wt%Al₂O₃–CaO–SiO₂–Fe₂O₃ slag by decreasing the amount of complex AlO₄ units and promoting the formation of simplified silicate monomers. The addition of Al₂O₃ to slags could promote the polymerization of the slag structure by increasing the quantities of complex AlO₄ tetrahedral and complicated Si–O units. Variations in the degree of structure polymerization showed similar trends at the same superheat degree and the same quenching temperature, and both samples could be used for analyzing the impact of Al₂O₃ on slag structures. Finally, the viscous behavior of the present slag system was evaluated. Increasing Al₂O₃ content could increase slag viscosity, and the apparent activation energy increased from 132.13 to 174.83 kJ/mol as the content of Al₂O₃ increased from 4.16wt% to 17.07wt%.