High-temperature experiments were carried out for the slag systems of “FeO”-SiO₂-CaO-Al₂O₃ and “FeO”-SiO₂-CaO-MgO at 1200 °C and p(O₂) of 10^-7 kPa. The equilibrated samples were quenched, and the phase compositions were measured by electron probe microanalysis (EPMA). A series of pseudo-ternary and pseudo-binary phase diagrams are constructed to demonstrate their applications in copper smelting process and evaluation of the thermodynamic database. Spinel and tridymite are identified to be the major primary phases in the composition range related to the copper smelting slags. It is found that the operating window of the smelting slag is primarily determined by w_Fe/w_SiO2 ratio in the slag. Both MgO and Al₂O₃ in the slag reduce the operating window which requires extra fluxing agent to keep the slag to be fully liquid. Complex spinel solid solutions cause inaccurate predictions of the current thermodynamic database.