To avoid slag sticking onto the ladle snorkel during the CAS-OB refining and alloying process, CaO-B2O3 and CaO-CaF2 were respectively employed as modifiers added into the ladle slag. The desulfurization capability of modified slag was investigated. The results of experiments on sulphur partition equilibrium between metal and slag indicate that the sulphur content of metal can be further decreased by the modified slag. When CaO-B2O3 is used as additive and the mass ratio of CaO to B2O3 is 1:1, the desulfurization ratio of slag to liquid steel is 50%, the final sulphur content can be controlled at 0.002%. The efficiency of desulfurization is fine. When CaO- CaF2 is employed as additive and the mass ratio of CaF2 to CaO is 2:8, the final sulphur content of metal can be controlled lower than 0.004%, the efficiency of desulfurization is also acceptable.