Abstract: An expression was regressed to describe the relation of phosphate capacity in slag to slag's optical basicity and temperature. An iso-phosphate capacity graph of the CaO-SiO₂-FeO (10% MgO) slag system was depicted by the regression expression. The effect of converter end-point slag, end-point composition and temperature on the phosphorus content in steel was analyzed. When the phosphate capacity in slag is constant, the distribution ratio of phosphorus between slag and steel increases with the converter final carbon content decreasing; but at a constant final carbon content, with the increase of phosphate capacity in slag, the distribution ratio of phosphorus between slag and steel increases. It is suggested that the final carbon content is controlled between 0.03% and 0.04%, the slag basicity is more than 3.5, the FeO content in slag is less than 18%, and the P₂O₅ content in slag is less than 2%, which are favorable for producing liquid steel with the final phosphorus content less than 0.008%.