An activity calculation model for SiO2-Al2O3-CaO-MgO-FeO-MnO slag system has been established according to the molecule-ion coexistence theory. From the model, the activity of FeO in the refining slag can be calculated, and the effects of MgO, FeO and basicity w(CaO)/w(SiO2) in slag composition on the activity of components related to FeO have been analyzed in this paper, which can provide reference for steelmakers to study the control of resulfurization at changing slag’s process of LF refining to blow tire cord steel.