Abstract: The influence of MgO content on liquid phase in CaO-SiO₂-Al₂O₃-MgO slag system was simulated by Factsage software.The results show that,with increase of MgO content in CaO-SiO₂-Al₂O₃-MgO slag,the low melting point liquid area moves forward ‘low CaO high SiO₂’ area. 1500℃ liquid area ratio of phase diagram rises from 25.05% of 0% MgO(mass fraction) to 52.69% of 9% MgO,and drops to 46.70% of 15% MgO. 1400℃ liquid area ratio of phase diagram rises from 14.41% of 3% MgO to 34.39%of 11% MgO,and drops to 31.04% of 15% MgO. 1300℃ liquid area ratio of phase diagram rises from 5.57% of 5% MgO to 11.02% of 14% MgO,and drops to 10.05% of 15% MgO. 1200℃ liquid area ratio of phase diagram is 0 when MgO content during 0 to 6%,and rises from 0.88% of 7% MgO to 1.22% of 11% MgO,then changes to 0 when MgO content during 13% to 15%.Simulation results provide an effective guidance for composition selection of steelmaking slag with the CaO-SiO₂-Al₂O₃-MgO components as the basic configuration.