Abstract: In order to improve the dephosphorization capacity and reduce the consumption of raw materials in steelmaking process, CaO-FeO-SiO₂-P₂O₅ composite phase slag with basicity R of 1.3, 1.5 and 1.8 was prepared in laboratory. The effect of basicity of composite phase slag system on dephosphorization ability under isothermal conditions was studied. The results show that under this experimental condition, the dephosphorization ability decreases first and then increases, when the dephosphorization time is 0-120 s, the dephosphorization ability is the strongest at 120 s. The apparent diffusivity of phosphorus in solid solution of CaO-FeO-SiO₂-P₂O₅ slag increases with the increase of basicity of slag system. Under the condition of R=1.3, 1.5 and 1.8, the diffusivity of phosphorus are 1.68×10^-13,2.79×10^-13,and 9.54×10^-13 m²/s, respectively. When the time of dephosphorization is same, the size of solid solution increases with the increase of diffusion coefficient.