The ternary system of K2SO4 + KCl + H2O at 303.15 K was studied by the method of isothermal solution saturation and wet residues. The solubility and density of the system were determined and the equilibrium solids were also investigated by the Schreinemakers method and verified by X-ray diffraction. On the basis of the experimental data, the phase diagram and the diagram of density versus composition were plotted. The ternary system had two uninvariant curves, one cosaturated point (K2SO4 + KCl), and two crystalline regions corresponding to K2SO4 and KCl. The crystallization region of K2SO4 was much larger than that of KCl. The system was a simple eutectic type, in which neither double salt nor solid solution was formed. The research can offer meaningful data support for industry and a forthcoming study. These data could be conducive to the processes of separation and crystallization in the chemical industry.