Solid-liquid phase equilibrium and physicocheinical properties (pH, n(D), p) for the ternary system KCl + KH2PO4 + H2O at (298.15 and 313.15) K were investigated by the method of isothermal solution saturation and Schreinemaker's wet residue. On the basis of the experimental data, corresponding phase diagrams and diagrams of physicochemical properties vs composition were plotted. In the phase diagrams, there are one eutonic point, two uninvariant curves, and two crystallization regions corresponding to KCl, KH2PO4. The phase diagrams of the ternary system show similar tendencies at different temperatures. At each temperature, the crystallization region of KH2PO4 is larger than that of KCl, and the crystallization region of KH2PO4 becomes larger as the temperature increases. Physico-chemical properties of the solid-liquid phase equilibrium solution vary regularly with the composition of KH2PO4 oncentration. The experimental data have been satisfactorily compared with literature data. The measured data and phase equilibrium diagrams for the ternary system can provide the fundamental basis for the industrial production of KH2PO4 by the method of extraction with industrial KCl and industrial H3PO4 aqueous mixtures.