Abstract: The effect of alkaline oxide content,optical basicity,total iron content in slag and temperature on the measured slag phosphate capacity was investigated through the calculation and analysis of the slag phosphate capacity during the converter dephosphorization process combined with previous study. The relationship between phosphate capacity and composition of slag was fitted according to the production data. In addition,different prediction models of phosphate capacity were compared and analyzed from previous studies. Based on the coexistence theory,the prediction model of phosphate capacity in slag system was established,and the error analysis shows that the prediction model is accurate and credible,which can provide a theoretical guidance for the field production.