Abstract: In order to meet the requirements of low sulphur content for ultra-low carbon steel, a new method is developed to inject flux via RH equipment or to add flux for desulphurization. Main methods of desulphuriza - tion and the requirements of the desulfurizer in RH vacuum degassing process are presented in this paper. The thermodynamics and kinetic are analyzed. Desulphurization mechanism of this method, selection and compo - nents of desulphurizing agent as well as electrical steel modification desulphurization of converter slag and some existing problems are also discussed. The prospect on the preparation and application trends of RH desulfurization process and desulfurizer is introduced.