Abstract: Based on the coexistence theory of slag structure,a calculating model of action concentration was established for the slag system CaO-MnO-SiO₂.The effects of basicity B and temperature on the action concentration of MnO,N_MnO,were analyzed.The results show that the calculated values of N_MnO are in good agreement with the measured activity values of a_MnO,indicating that this calculating model can wholly embody the characteristics of the slag system.N_MnO increases with the basicity when B<2.0,and reaches the maximum value when B is 2.0,then decreases with the basicity when B>2.0.In the range of low basicity,the influence of basicity on N_MnO is more apparent.No obvious influence of temperature on N_MnO was observed.