Phase equilibrium in the system KH2PO4-NaH2PO4- H2O at 288.15 and 318.15 K was determined using methods of isothermal solution saturation and moist residues. The equilibrium phase solid was also verified by X-ray diffraction (XRD). Furthermore, the crystallization areas in the phase diagram were analyzed and discussed. In the phase diagram of the two temperatures, there is one invariant point, two single saturated liquid curves, and two crystallization fields corresponding to the single salts NaH2PO4 center dot 2H(2)O, KH2PO4 at 288.15K and NaH2PO4 center dot H2O, KH2PO4 at 318.15K The density of saturated solutions for the systems studied at 288.15 and 318.15 K was measured and calculated to test the precision of the chosen model. This research filled the blank of phase diagram in this ternary system and can be applied to the production of potassium dihydrogen phosphate process to optimize the crystallization and separation process of potassium dihydrogen phosphate.