Abstract: In order to study the reaction rate of RH vacuum decarburization and its influencing factors, and effectively control the change in carbon content of ultra-low carbon steel in the RH vacuum treatment process, a mathematical model of RH vacuum decarbnrization was established on the basis of the kinetic and thermodynamic theories. A relationship between the reaction depth of internal decarburization and the decarburization reaction rate was studied by the RH vacuum decarburization model. The calculated results of this model show that the change in reaction depth is corresponding to the decarbnrization reaction rate. Taking a pre-vacuum operation can increase the reaction depth, decrease the impact of the early decarbonization turning point and accelerate the decarburization reaction in the early stage. The critical point of time from internal to surface decarbonization is found out in the late of RH treatment.