Abstract: On basis of the quaternary slag system used for bearing steel refining,the influence of basicity,Al₂O₃ and MgO content on the viscosity of CaO-MgO-Al₂O₃-SiO₂ slag system has been investigated by Fact Sage software,and the calculation was compared with the measured value for actual system,for the purpose of providing theoretical basis for optimization of the refining slag system components. Viscosity of the slag system could be simulated with the thermodynamic software. There was a tendency that viscosity declined to the minimum and then increased with increase of the basicity. When the basicity is low,the addition of basic oxides accelerates disintegration of silicate minerals,then the amount of α-C₂S crystallization and the viscosity of the slag decline dramatically. When the basicity is high,the amount of two kinds of solid solution,in which the main component is CaO and MgO,respectively,increases gradually and then the viscosity increases,suggesting that viscosity of the slag system declines with Al₂O₃ content increasing. When the content of Al₂O₃ is low,the amount of α-C₂S and the solid solution crystallization decline with the increase of Al₂O₃ content,and the viscosity declines quickly. At the high Al₂O₃ level,the amount of solid phase is low,change of Al₂O₃ content has weak influence on the viscosity. When the temperature of slag system is low,the viscosity has been impacted by the modification of MgO on net structure of the molten slag and the crystallization of solid phase. There is a tendency that the viscosity declines to the minimum and then increases with the increase of MgO content. When the temperature of the slag system is high,the viscosity is controlled by the amount of solid solution crystallization and increases with the increase of MgO content.