In this study, the solubility and density of a ternary system (KH2PO4 + KCl + H2O) at (288.15 and 303.15) K were determined; using isothermal,solution Saturation and wet residues. The equilibrium solids were detected by the Schreine-maker's method of wet residues and verified by X-ray diffraction (XRD). According to the experimental data, the diagram, of density versus composition and the phase diagram Were plotted. The crystallization regions of KH2PO4 and KCl were determined, and the crystallization region of KH2PO4 was much larger than that of KCl. The density of the saturated solution was calculated, and the model precision was tested. All results can offer fundamental basis for crystallization and separation processes in chemical industry.