To provide theoretical data for the hydrometallurgical process of lead-zinc ores and the separation process of secondary lead recovery, the quaternary system MgCl2-PbCl2-ZnCl2-H2O at 323 K was investigated using the isothermal dissolution equilibrium method. The isothermal solubility data were obtained, based on which the corresponding dry salt phase diagram and the water content diagram were constructed. The double salts MgZnCl4 center dot 6H(2)O and 3MgCl(2)center dot 2PbCl(2)center dot 18H(2)O were found in the equilibrium solid phases as its ternary subsystems. The dry salt phase diagram contains three invariant points, seven isothermal solubility curves, and five crystallization fields (the crystal salts are PbCl2, 3MgCl(2)center dot 2PbCl(2)center dot 18H(2)O, ZnCl2, MgZnCl4 center dot 6H(2)O, and MgCl2 center dot 6H(2)O(Bis), respectively), and the crystallization region of PbCl2 is the largest. At the same time, the invariant data of this quaternary system and its ternary subsystems at different temperatures were compared and discussed.