Abstract: In order to investigate the influence 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 influence of free basicity (Ro), w(MgO) and w(Al2O3) on the desulfurization and alkali removal ability of blast furnace slag was investigated through orthogonal experiments and the five-element pseudo-ternary phase diagrams of different components of blast furnace slag system calculated and drawn by using the Factsage 7.1 thermodynamic simulation software, and the appropriate control range of basicity (Ro), w(MgO) and w(Al2O3) in the slag were given in combination with the production practice. The results show that:?(1) free basicity (Ro) is the most significant factor affecting the slag desulfurization and alkali removal ability, free basicity (Ro) is the most significant factor affecting the slag desulfurization and alkali removal ability, with the increase of Ro, the concentration of O2? in slag increases, which leads to the disintegration of Si?O, the viscosity of slag decreases, and the mass transfer between slag and metal liquid is promoted, which makes S2? easier to migrate into slag, the thermodynamic and kinetic conditions of slag desulfurization are improved, thus the desulfurization ability is improved. the appropriate Ro should be controlled within the range of 1.05-1.15. (2) w(MgO) is a secondary factor affecting the slag desulfurization ability, with the increase of w(MgO), the fluidity and stability of the slag are improved, which is beneficial to improve the kinetic conditions of slag desulfurization and can reduce the activity of (K2O+Na2O) in the slag, thus the alkali removal ability is improved. The appropriate w(MgO) should be controlled at about 15 %. (3) w(Al2O3) is a secondary factor affecting the alkali removal ability of blast furnace slag, with the increase of w(Al2O3) , high melting point materials such as MgAl2O4 are easily formed, which increases the consumption of free oxygen ions in the slag and 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 will lead to the increase of slag viscosity. The appropriate w(Al2O3) should be controlled at about 12 % .