A mathematical model was established and applied to simulate the decarburization of RH-MFB process in Pansteel Company. The study of the effects of w[C]0, w[O]0, Ar flowrate, evacuation rate and MFB lance blowing parameters on the decarburization process was carried out. The results show that this model can give the quantitative understanding of the process, especially the behavior of MFB lance blowing. This model has realized the optimum process of RH-MFB refining for ultra-low carbon steels in Pansteel.