The mass transfer behavior of phosphorus in the multiphase dephosphorization slag at 1623 K (1350°C) is investigated in this article by adding the reagent P 2 O 5 powders into the CaO-FeO t -SiO 2 slag saturated with a directly precipitated 2CaOAESiO 2 phase. The results show that the phosphorus was first quickly dissolved and transferred from the liquid slag to the precipitated solid 2CaOAESiO 2 particle; then it reacted with the solid 2CaOAESiO 2 particle to form (2CaOAESiO 2 -3CaOAEP 2 O 5 ) solid solution at the edge of a large solid particle. The further diffusion of phosphorus from the formed (2CaOAESiO 2 -3CaOAEP 2 O 5 ) solid solution reaction layer into the core of the 2CaOAESiO 2 particle is the limiting step during the enrichment-phosphorus process. It is observed that the enrichment-phosphorus process is changed when the amount of added P 2 O 5 content is increased and the formed solid solution phase is torn and peeled off from the surface of the 2CaOAESiO 2 particle. In addition, the study shows that the solid solution structure is changed and becomes more compacted with the addition of P 2 O 5 content, leading to the increment of solid solution density.