Introduces the hot metal desulphurization process with granulated magnesium at Lengshuijiang Iron & Steel Co. Ltd. Analyzes its desulphurization mechanism. Studies the relations between desulphurization capacity and magnesium consumption and the factors affecting the desulphurization rate as well as desulphurization cost. Puts forward that optimizing desulphurization parameters and applying new technology actively are the key to improve economic benefits.