According to the coexistence theory of slag structure, phasediagram Na
2O-SiO
2 and the corresponding thermodynamic data, a calculating model of mass action concentrations for the slag system has been deduced. The calculated
NNa2O and
NsiO2 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 Na
2O-SiO
2 too.