The solubility and the physicochemical properties corresponding to densities and pH values in the solid-liquid system (NaCl-KCl-CaCl2-H2O) at 303.15 K have been studied with the method of isothermal dissolution equilibrium. The diagrams of dry-salt phase, water, and physicochemical properties versus composition in the system were plotted. The dry-salt phase diagram of the system includes one three-salt cosaturated point, three isotherm dissolution curves, and three crystallization regions corresponding to sodium chloride, potassium chloride, and calcium chloride tetrahydrate. Neither solid solution nor double salts were found. On the basis of the extended HW model and its temperature-dependent equation, the values of the Pitzer parameters of beta((0)), beta((1)), and C-phi of NaCl, KCl, and CaCl2, the mixed ion-interaction parameters of theta(Na,K), theta(Na,Ca,) theta(K,Ca), psi(Na,K,Cl), psi(Na,Ca),(Cl), psi(K,Ca,Cl), Debye-Huckel parameter A?, and the solubility equilibrium constants of the solid phase in the quaternary system at 303.15 K were obtained, and the solubilities of the mentioned quaternary system at 303.15 K were calculated. The results show that the predicted solubilities agree well with the experimental data.