Abstract: The phosphorus in P-bearing converter slag is mainly existed in the form of n2CaO·SiO₂-3CaO·P₂O₅ (for short nC₂S-C₃P) solid solution in the P-rich phase. For a better understanding of the phase in the CaO-SiO₂-Fe₂O₃-P₂O₅ slag system, and an effective enrichment of the phosphorus in converter slag, the effect of MgO and MnO on the P₂O₅ content in the P-rich phase and effect on crystallization phase were studied by performing thermodynamic analysis. The results show that the content of P₂O₅ in the P-rich phase increases by adding of MgO and MnO to the CaO-SiO₂-Fe₂O₃-P₂O₅ slag system. However, the increasing of MgO and MnO has no effect on the P₂O₅ content. With the addition of MgO and MnO, the iron rich phase is observed in an increasing tendency, which shows that it is favorable to the magnetic separation of P-bearing slag.