Abstract: A predominantly liquid and MgO saturated slag is preferred in secondary metallurgical processes.It contributes to the mass transfer in molten steel and the extension of the refractory life.In the present study, the saturability of Mg O in quaternary Ca O-Si O₂-Al₂O₃-MgO system in secondary metallurgical slag with low basicity was investigated by commercial thermodynamic software and laboratory experiments.The result shows that in the basicity (Ca O/Si O₂) range of 0.5-1.8, the saturability of MgO fluctuates between2%and 39%and decreases with the increase of basicity.The saturability of MgO increases in the MgO saturated phase periclase (MgO) and forsterite (2MgO·Si O₂) and decreases in the MgO saturated phase spinel (MgO·Al₂O₃) with an increase in Al₂O₃content.Si O₂content has a great effect on the saturability of MgO in lime-silicate slag.Besides, the MgO saturability increases with an increase in temperature.