According to the coexistence theory of slag structures, the calculating model of the oxidizing capability for CaO-MgO-FeO-Al₂O₃-SiO₂ refining slag system has been deduced. Based on this model, the concept and the calculating method of the optimum slag basicity are put forward. The results have proved that at this basicity,steel with lower oxygen content can be obtained and non-metallic indigenous globular inclusions can be avoided more effectively than at other basicity.