The Cu-Mg-Zn system is one of the important ternary sub-systems in multicomponent light alloys. This ternary system was reassessed in the framework of the CALPHAD method in view of the drawbacks of previous thermodynamic assessments. A critical review of the literature data on phase equilibria and thermodynamic properties has been performed. An updated thermodynamic description for the ternary system is obtained by considering the available literature data. Isothermal and vertical sections, as well as the liquidus surface and the related invariant reactions, are calculated and compared with the corresponding experimental ones. The reaction scheme for the whole ternary system is also presented. The solidification microstructures of several Cu-Mg-Zn alloys along the Mg-Cu0.5Zn0.5 and Mg-Cu0.385Zn0.615 sections were simulated under equilibrium and non-equilibrium conditions. The calculated solidification microstructures do not contradict the experimental ones presented in the literature. This indicates the reliability of the thermodynamic parameters.