Abstract: According to the coexistence theory of slag structure, phasediagram Na₂O-SiO₂ and the corresponding thermodynamic data, a calculating model of mass action concentrations for the slag system has been deduced. The calculated N_Na2O and N_siO2 by this model are in good agreement with measured a_Na2o and a_sio2 from literature source respectively. Thus it shows that the coexistence theory of slag structure is applicable to slag system Na₂O-SiO₂ too.