Abstract: A mathematical model was developed to investigate the molten steel flow and decarburization process in RH vacuum degasser system under the action of electromagnetic field. A modified homogeneous flow model was introduced to calculate the flow field in RH degasser, thus getting rid of some earlier methods that are unavailable to be generalized, such as reducing the computation region or presetting the distribution of gas volume fraction. The RH system with and without rotating magnetic field were simulated, and the numerical results showed that the flow field and decarburization process are in agreement with the experiment results in some earlier works. At the early stage of the decarburization reaction, when magnetic flux density is 0.04T the decarburization rate of RH system with electromagnetic stirring is 15% higher than that without electromagnetic stirring because the vortex due to the stirring enhances the stirring power hence the decarburization rate. Finally, the carbon contents under different magnetic flux densities tend to the same stable value.