The desulfurization ability of refining slag with relative lower basicity(B) and Al2O3 content(B = 3.5-5.0; 20wt%-25wt% Al2O3) was studied. Firstly, the component activities and sulfide capacity(CS) of the slag were calculated. Then slag-metal equilibrium experiments were carried out to measure the equilibrium sulfur distribution(LS). Based on the laboratorial experiments, slag composition was optimized for a better desulfurization ability, which was verified by industrial trials in a steel plant. The obtained results indicated that an Mg O-saturated Ca O-Al2O3-Si O2-Mg O system with the basicity of about 3.5-5.0 and the Al2O3 content in the range of 20wt%-25wt% has high activity of Ca O(aCa O), with no deterioration of CS compared with conventional desulfurization slag. The measured LS between high-strength low-alloyed(HSLA) steel and slag with a basicity of about 3.5 and an Al2O3 content of about 20wt% and between HSLA steel and slag with a basicity of about 5.0 and an Al2O3 content of about 25wt% is 350 and 275, respectively. The new slag with a basicity of about 3.5-5.0 and an Al2O3 content of about 20wt% has strong desulfurization ability. In particular, the key for high-efficiency desulfurization is to keep oxygen potential in the reaction system as low as possible, which was also verified by industrial trials. The desulfurization ability of refining slag with relative lower basicity (B) and Al2O3 content (B = 3.5-5.0; 20wt% -25wt% Al2O3) was studied. Firstly, the component activities and sulfide capacity (CS) of the slag were calculated. Then slag-metal equilibrium experiments were carried out to measure the equilibrium sulfur distribution (LS). Based on the laboratorial experiments, slag composition was optimized for a better desulfurization ability, which was verified by industrial trials in a steel plant. that an MgO-saturated CaO-Al2O3-Si02-MgO system with the basicity of about 3.5-5.0 and the Al2O3 content in the range of 20wt% -25wt% has a high activity of CaO (aCaO), with no deterioration of CS compared with conventional desulfurization slag. The measured LS between high-strength low-alloyed (HSLA) steel and slag with a basicity of about 3.5 and an Al2O3 content of about 20 wt% and between HSLA steel and slag with a basicity of about 5.0 and an Al2O3 content of about 25 wt% is 350 and 275, respectively. The new slag with a basicity of about 3.5-5.0 and an Al2O3 content of about 20 wt% has a strong desulfurization ability. in the reaction system as low as possible, which was also verified by industrial trials.