Abstract: To investigate the effect of slag composition on desulfurization and alkali removal ability of blast furnace slag for smelting Bayan Obo ore, based on the actual composition of blast furnace slag, the effect of free basicity (R_o), w(MgO), and w(Al₂O₃) on the desulfurization and alkali removal ability of blast furnace slag was investigated by performing orthogonal experiments and on the basis of five-element pseudoternary phase diagrams of various components of a blast furnace slag system calculated and drawn using Factsage 7.1 thermodynamic simulation software, and the appropriate control range of R_o, w(MgO) and w(Al₂O₃) in the slag were given in combination with the production practice. The results show that: R_o is the most significant factor affecting slag desulfurization and alkali removal ability. With the increase in R_o, the O²⁻ concentration in slag increases, resulting in Si−O disintegration, and slag viscosity decreases. In addition, the mass transfer between slag and metal liquid is accelerated, which makes S²⁻ easier to migrate into slag, the thermodynamic and kinetic conditions of slag desulfurization are improved, thus improving the desulfurization ability. The appropriate R_o should be controlled within the range of 1.05–1.15. w(MgO) is a secondary factor affecting the slag desulfurization ability. With the increase in w(MgO), the fluidity and stability of the slag are improved, which are beneficial for improving the kinetic conditions of slag desulfurization and reducing the activity of (K₂O+Na₂O) in the slag, thus improving the alkali removal ability. Appropriate w(MgO) should be controlled at approximately 15%. w(Al₂O₃) is a secondary factor affecting the alkali removal ability of blast furnace slag. With the increase in w(Al₂O₃), high melting point materials such as MgAl₂O₄ are easily formed, thereby increasing the consumption of free oxygen ions in the slag. This increase is not conducive to the improvement of desulfurization reaction kinetic conditions. Although increasing w(Al₂O₃) is beneficial for removing alkali, high w(Al₂O₃) is not conducive to desulfurization and leads to an increase in slag viscosity. Appropriate w(Al₂O₃) should be controlled at approximately 12%.