With the consumption of high-quality iron ore resources, the grade of iron ore available to iron and steel enterprises gradually decreases. Therefore, high alumina iron ore resources have attracted more and more attention. However, high alumina materials in the smelting process in the blast furnace will bring a series of problems, such as viscous slag iron, low furnace temperature, and smelting safety issues. From the perspective of blast furnace slag, this paper analyzes the effects of Al₂O₃ mass fraction on equilibrium phase, melting temperature and phase precipitation temperature of blast furnace slag, as well as the changes of liquid phase zone and viscosity of high alumina slag by using FactSage thermodynamic software. The results show that 1) when the slag is of low alumina (5% to 10%), with the increase of Al₂O₃ content, the melting temperature of slag increases, and the precipitates are feldspar phase and pure material phase. The viscosity of blast furnace slag increases slowly. The content of SiO₂ in slag is large, and the viscosity of slag is too high, which is not suitable for blast furnace smelting. 2) When the slag is of medium alumina (10% to 15%), with the increase of Al₂O₃ content, the melting temperature of the slag increases, and the precipitates are spinel phase, feldspar phase and pure material phase. The viscosity of the blast furnace slag increases slightly when the content of alumina is low. The Al₂O₃ content has little effect on the properties of the blast furnace slag, and the increase of the binary basicity of the slag has an obvious effect on the decrease of the viscosity of the blast furnace slag. 3) When the slag contains high alumina (15% to 30%), with the increase of Al₂O₃ content, the melting temperature of the slag increases, the precipitated phases are spinel phase, feldspar phase and pure material phase. The viscosity of the blast furnace slag increases at a rate of 0.011 Pa·s/%. In order to ensure that the solid phase precipitation temperature and viscosity of the slag meet the normal blast furnace smelting requirements, it is suggested that MgO/Al₂O₃ should be in the range from 0.4 to 0.5, the binary alkalinity R₂ range should be between 1.1 and 1.3, and the mass fraction of TiO₂ can be appropriately increased to about 5% in neutral atmosphere.