A Pitzer-Simonson-Clegg model has been applied to calculate the isotherms of the system KNO3-LiNO3-NaNO3-H2O and its subsystems. The model parameters are fitted against experimental solubility and water activity in the sub-binary and sub-ternary systems of the titled quaternary system. The solubility of the system KNO3- LiNO3-NaNO3-H2O at 298.1 and 308.5 K and its subsystems KNO3-LiNO3-H2O at 283.1 K and LiNO3-NaNO3-H2O at 323.1 K are elaborately measured for verifying the reliability of the model prediction. Comparisons indicated that the calculated values are in agreement with our experimental data and literature data.