Water activities for the CaCl2-SrCl2-H2O ternary system and its sub-binary system were elaborately determined using an isopiestic vapor-pressure method at 323.15 K. The solubility of the titled ternary system at 323.15 K was also determined by the isothermal equilibrium method. The Pitzer-Simonson-Clegg (PSC) model was used for correlating the determined water activity and solubility data, simulating the thermodynamic properties, and predicting the solubility isotherm of the titled system. The reliability of the determined solubility results in this work was evaluated by comparing with the solubility isotherms calculated with the PSC model. The model simulation results showed that the thermodynamic properties and solubility can be predicted with binary parameters only. The experiment and simulation results showed that the equilibrium solid phase in the CaCl2-SrCl2-H2O ternary system are pure salts, and no solid solution was found at 323.15 K.