A thermodynamic model for calculating phosphate capacity of CaO-FeO-MgO-SiO2-MnO-TiO2-V2O5-P2O5 slags has been developed based on the IMCT. In this paper, the effects of slag basicity, (%FeO), (%MgO) (%MnO), (%TiO2) and (%V2O5) on phosphate capacity were studied. The results illustrate that there is a parabola relationship between the calculated lgCP and slag basicity, (%FeO), (%TiO2) and (%V2O5), and the increase of slag basicity and (%FeO) will promote the lgCP, conversely, the increase of (%TiO2) and (%V2O5) will decrease the lgCP. Meanwhile, lgCP has a negative linear relationship with (%MgO) and (%MnO). Moreover, the model can predict not only the total phosphate capacity CP of CaO-FeO-MgO-SiO2-MnO-TiO2-V2O5-P2O5 slags, but also the respective contribution of CP,i of nine dephosphorization products, which the generated 3CaO·P2O5 accounts for 98.93% of contribution to the total phosphate capacity and 4CaO·P2O5 for 1.06%, 2CaO·P2O5 for 0.0016%, respectively, while the other structural units only have a negligible contribution.