Abstract: In order to present the operating principles and the appropriate control objectives for Nan Steel,according to dephosphorization theory and characteristics of double slag dephosphorization in combined blown converter,together with the conditions of molten iron in Nan Steel,the influences of silicon content,temperature and phosphorus content on dephosphorization were analyzed and the reasonable smelting solution was presented. The result shows that it can achieve the dephosphorization effect that phosphorus content is less than 4×10^-5 and the dephosphorization rate is 97.3%,by using the optimizing process operation of slagging,oxygen blowing,first deslagging and end-point control,in Nan Steel 120 t combined blown converter. The need of ultra-low phosphorus steel produce can be met through the optimized solutions.