Using the metastable equilibrium solubilities data of the ternary systems at 298 K, the single salt parameters and mixing ion-interaction parameters of Pitzer’s equation were fitted by multiple linear regression method. The solubilities of metastable phase equilibrium of the quaternary system KK^+/Cl^-,CO(3)^(2-),B(4)O(7)^(2-)–H(2)O at 298 K were calculated by Pitzer’s equation. The results showed that the calculated value has a good agreement with the measured data.