Abstract: The existence of sulfur in steel slag restrains its recycle and re-use in steelmaking process.To recycle the waste slag as much as possible,a novel approach of two-step roasting desulfurization has been proposed in this present study.The thermodynamic analysis shows that CaO stable region in Ca-S-O system increases with increasing of reaction temperature and decreasing of oxygen partial pressure in atmosphere.So,the roasting process should be controlled at a high temperature and a low oxygen pressure.The optimized parameters of two-step roasting are also obtained.In the first stage,CaS is roasted under the air atmosphere at 900 ℃ for 180 min,then the intermediate is reacted under argon gas at 1 100 ℃ for 40 min.After treatment of two-step roasting,the sulfur content in the actual waste slag was decreased from 0.8% to 0.23%.The products can be recycled to the steelmaking process.