Abstract: The admixture of CaO and Mg was used to treat the hot metal for desulphurizing in Chongqing Steel Corporation. The fluidity of the desulfurizd slag is not of benefit to separating the molten slag from hot metal. Usually, the way to resolve the problem is modifying the desulfurizd slag with using the modifying agent. The compositions of the modifying agent were optimized through experiment analysis. It is found that the melting point of the modifying agent is 930℃, and the viscosity is 0.627 Pa·s at 1250℃, the melting point of the modified desulfurization slag is from 1350 to 1370℃, and the viscosity is less than 1 Pa·s. The primary plant trial showed that the modifying agent could properly solve the problem of separating slag from hot metal.