Abstract: According to the phase diagram and the coexistence theory of slag structure,a calculating model of mass action concentrations for CaO-Al₂O₃-SiO₂ slag system is formulated.Satisfactory agreement between calculated and measured values shows that this model can reflect the structural characteristics of this slag system.Meanwhile,It is shown that the bascity at which 2CaO·SiO₂ forms increases with the increase of Al₂O₃ content and that there is an active zone where ∑n is smaller than other part of the slag system.