Abstract: In order to improve the dephosphorization effect during slag-remaning and double slag process，the influence of slag basicity and FeO，MgO and MnO content on lg LP（LP is the equilibrium distribution ratio of phosphorus in slag- steel）were calculated by thermodynamic empirical formula. The results indicate that the theoretical composition range of slag during dephosphorization process is that slag basicity is about 1.5~2.5， w（FeO）=15%~25%，w（MgO）＜8%，w（MnO）＜10%. Liquidus diagram of CaO-SiO2-FeO-MnO-MgO slag systems was calculated by thermodynamic software Factsage. Theoretical calculate slag forming route was designed for dephosphorization stage during slag-remaining and double slag process with the comprehensive consideration of empirical formula and liquidus diagram calculate results. The characteristics of the slag forming route is that FeO content is high and the slag basicity changes from high to low and then increases slowly. The production practice results indicate that the consumption of slag material decreased by 22.7% and dephosphorization rate reached 92.2% at the end of converter steelmaking with this slag forming route. The experiment achieved the dual goals of decreasing slag material consumption and efficient dephosphorization.

Key words: Slag-remaining and double slag process, dephosphorization, slag forming route