Laboratory study was carried out on deep desulfurization of molten steel by CaO-Al2O3 based refining slag containing BaO at 1873K to achieve lower sulphur level in steel. A mathematical model of desulfurization was established with the methodology of quadratic orthogonal regression. According to the modle, the influence of A12O3/CaO, MgO, CaF2, BaO in slag on desulfurization rate was analyzed. The results showed that the desulfurization rate almost linearly decreased with the increase of A12O3/CaO, and it increased firstly then decreased when MgO and BaO content increased respectively, yet the trend is on the contrary as CaF2 content increased．Based on the comprehensive analysis, the optimum composition of the refining slag was achieved under the experimental conditions.