The solubility and phase diagram of the CsCl-MgCl2-H2O ternary system, at a relatively wide temperature range, are significantly important for the extraction of the cesium resources from salt lakes. The isothermal dissolve equilibrium (IDE) method was used to determine the solubility of this ternary system at T = 273.15, 308.15, and 348.15 K, which have not been reported yet. The measured results indicated that the equilibrium solid phases in this ternary system from T = 273.15 to 348.15 K are CsCl, double salt CsCl center dot MgCl2 center dot 6H(2)O, and MgCl2 center dot 6H(2)O, and correspondingly there are three crystal regions and two cosaturated points for these solid phases. The solubility of the salt increased with the increase of the temperature, and the double salt CsCl center dot MgCl2 center dot 6H(2)O and CsCl crystal regions also became larger with the increasing temperature. However, the crystal regions of the MgCl2 center dot 6H(2)O solid phase are not enlarged obviously. Based on the solubility and phase diagram of CsCl-MgCl2-H2O at different temperatures, the separation process of CsCl resources has been proposed.