A mathematical model for the combined top and bottom blowing VOD refining of stainless steel has been proposed. It is essentially based on the assumptions made for the AOD process of stainless steel under the conditions of combined side and top blowing in our previous work, but considering the effects of reduced ambient pressure and the degassing in the refining. The influence of the operational factors, the non-isothermal conditions and others has all been considered. The model has been applied to 20 heats of 409L-grade ferritic stainless steel refining in a 120 t VOD vessel. The results show that the changes in the composition and temperature of the steel during the whole refining process with the time can be precisely predicted by the model. The critical carbon concentration (after which the decarburization alters to be limited by the mass transfer of carbon in molten steel) for the combined blowing of 409L-grade steel in this work is in the range of 0.119 to 0.182 mass%, in agreement essentially with that of the oxygen top blowing. The influences of the related factors on the refining effectiveness and the optimization of blowing technology have been examined from the model predictions. The model can offer useful information for determining and optimizing the technology of the VOD refining of stainless steel, and controlling the process in real time and online.