The Ca(NO3)(2)-Mg(NO3)(2)-H2O ternary system is a prospective system for heat storage, and its polythermal phase diagram is the basis for phase change materials design. However, the existing experimental data are dispersed and inconsistent with each other. In this work, we elaborately determined solubility isotherms of the ternary system at T = (273.15, 298.15, and 323.15) K by an isothermal equilibrium method, and chose a Brunauer-Emmett-Teller model to simulate and to construct the phase diagram of the ternary system from (273.15 to 373.15) K. The determined phase diagram indicates that there are two stable solubility isotherms at (273.15 and 298.15) K corresponding to solid phase Ca(NO3)(2)center dot 4H(2)O and Mg(NO3)(2)center dot 6H(2)O. No stable solubility isotherm for solid phase Ca(NO3)(2)center dot 3H(2)O at (273.15 and 298.15) K has been found, different from the results reported in several references.