Abstract: Based on the ion and molecule coexistence theory,the calculating model of action concentration for CaO-MgO-FeO-SiO₂-P₂O₅ slag was established. Based on this,the influence of slag compositions on the Gibbs free energy of decarburization and dephosphorization was analyzed,the feasibility of dephosphorization and protection carbon was proved.On the basis of thermodynamic analysis,the high temperature thermal experiment was conducted and the result showed that when the slag basicity is about 5 and the（FeO） content is between 10% and 20% with the condition that initial carbon content is 0.5% and the phosphorus content is 0.032 5%,the dephosphorization rate is up to 75%,the carbon conservation rate is between 50% and 70%.