Fully utilizing the solid in the slag is relatively helpful to improve the dephosphorization capacity of the slag. There are a large number of 2CaO?SiO2 solid phases in the actual steelmaking slag, and the steelmaking operation is usually carried out under the condition of dicalcium silicate saturation. However, the effect of the existing 2CaO?SiO2 in the slag on the formation of the solid solution has not been well clarified. In this work, the dissolution behavior of dicalcium silicate and the formation of phosphorus-containing solid solution phase in CaO-FeO-SiO2-P2O5 system BOF (basic oxygen furnace) slag were studied by putting dicalcium silicate of the average size of about 450 μm into the slag at 1400℃. The results showed that three regions were formed around the dicalcium silicate particles, namely the unmelted dicalcium silicate region, the coexistence region of the liquid phase and the solid phase (2CaO?SiO2 solid phase and 2CaO?SiO2-3CaO?P2O5 solid solution), and the substrate slag layer. The formation mode of the phosphorus-containing solid solution phase in converter slag was the coexistence of the precipitation mechanism and diffusion mechanism. The phosphorus content in the solid solution formed by the precipitation mechanism was high and the content of other elements was low. The phosphorus content in the solid solution formed by the diffusion mechanism was relatively low and the content of other elements was relatively high. The dissolution of dicalcium silicate particles and the formation of a solid solution showed that after the dicalcium silicate particles were added to the slag, Fe2+ and PO43- in the slag diffused to the dicalcium silicate particles, and Ca2+ and SiO32- in the dicalcium silicate diffused to the slag, forming a solid solution of 2CaO?SiO2-3CaO?P2O5; After that, dicalcium silicate particles were further dissolved and completely transformed into 2CaO?SiO2-3CaO?P2O5 solid solution. With the extension of time, the solid solution particles gradually grew up.