The metastable equilibrium of the system contained with lithium, potassium, magnesium, and chloride in aqueous system was investigated at 323 K using an isothermal evaporation method. The isothermal experimental data and physicochemical properties, such as density and refractive index of the equilibrated solution, were determined. With the experimental results, the stereo phase diagram, the projected phase diagram, the water content diagram and the physicochemical properties versus composition diagrams were constructed. The projected phase diagram consists of three invariant points, seven univariant curves and five crystallization fields corresponding to single salts potassium chloride (KCl), lithium chloride monohydrate (LiCl·H2O), bischofite (MgCl2·6H2O) and two double salts lithium carnallite (LiCl·MgCl2·7H2O) and potassium carnallite (KCl·MgCl2·6H2O). Salt KCl has the largest crystallization region; it contains almost 95% of the general crystallization field.