The CaO–MgO–Al2O3 system is one of the most fundamental oxide systems for various metallurgical and ceramic applications. The composition of slag co-saturated by CaO and MgO is important for accurate slag control in the secondary refining steelmaking process. In the present study, the co-saturated slag compositions at 1550 °C to 1650 °C were determined from the classical quenching experiment with Pt crucible. Based on the new experimental data and available data in literature, thermodynamic re-assessment of the CaO–MgO–Al2O3 system was carried out and new phase diagrams were calculated in the ternary system.