Abstract: According to the coexistence theory of slag structure as well as the measured viscosities of CaO -MgO-CaF₂-SiO₂ -Al₂O₃ at different temperatures and compositions,calculating models of mass action concentrations for CaO -MgO-CaF₂-SiO₂-Al₂O₃, melts and viscosity for these melts have been formulated. The results of calculation agree well with practice, showing that these models can reflect the structural reality of the melts and the relatonship between viscosities as well as the mass action concentrations and temperatures of the melts.