The phase equilibrium of (KCl + K2SO4 + H2O) and (KH2PO4 + K2SO4 + H2O) at 288.15 K is studied by isothermal solution saturation method and wet residues. The equilibrium solid phases are analyzed by the Schreinemaker's method of wet residues and verified by X-ray diffraction (XRD). On the basis of the experimental data, the diagram of density versus composition and the phase diagram are plotted. The crystallization regions are determined. In the two systems, KCl and KH2PO4 have a strong salting-out effect on K2SO4, and the salting-out effect of KCl on K2SO4 is stronger than that of KH2PO4 on K2SO4. There are in all two crystalline regions, two univariant curves, and one invariant point. The crystalline region of K2SO4 is the largest, whereas that of KCl is the smallest. All results can offer fundamental data support for optimizing processes and further theoretical studies.