This paper calculated the phase diagrams of nine ternary systems including K+, Mg2+, Na+, Cl- and NO-3 and their binary systems by using a Pitzer-Simonson-Clegg thermodynamic model. The binary model parameters were fitted with experimental water activity and osmotic coefficient data, and the ternary model mixture parameters were obtained by regressing the experimental solubility data of the ternary salt-water systems. All model parameters and equilibrium constants of the solid phase were expressed as the functions of temperature. Furthermore, the positions of invariant points over the range of temperature from 273.15 K to 364.15 K of the ternary system KCl-KNO-3-H2O were accurately predicted, and the predicted vapor pressures over aqueous solutions which are saturated with the invariant points of this system were in good agreement with literature data. And the water activities of the saturated solution of the ternary systems KNO3-NaNO3-H2O and NaCl--NaNO3-H2O at 363.15 K were predicted and compared with experimental data. On the basis of these, we neglected the quaternary model parameters and just used the binary and ternary parameters to predict the isotherms of the quaternary systems K+, Mg2+∥Cl-, NO-3-H2O, K+, Na+∥Cl-, NO-3-H2O, Mg2+, Na+∥Cl-, NO-3-H2O and K+, Na+, Mg2+∥NO-3-H2O, and to predict the vapor pressure of saturated solution of the system K+, Na+∥Cl-, NO-3-H2O over the temperature range from 278.15 K to 323.15 K. The predicted values were in good agreement with experimental data.