KH2PO4 is widely applied in industrial material, agricultural fields, pharmaceutical, and food industries. To develop a new process to produce more purified KH2PO4 based on the extraction technology process reported before, SLE data of the reciprocal quaternary system (Na+, K+//SO42-, H2PO4-- H2O) at 313.15 K is investigated with isothermal solution saturation. Solubilities and physic-chemical properties (n(D), eta, pH) of saturated solutions are obtained. And corresponding phase diagram and physic-chemical properties are given. The phase diagram contains three invariant points, seven isothermal univariant curves and five crystallization zones, which refers to K2SO4, Na2SO4 center dot 3K(2)SO(4) (Gla), Na2SO4, KH2PO4, NaH2PO4 center dot 2H(2)O, respectively. The crystallization zone of salt Gla occupies the largest part, indicating it has the lowest solubility and can be more easily separated from the mixing solution in this system at 313.15 K. All physic-chemical properties (nu, eta, pH) of the system change regularly with changing composition of the saturated solution. And the phase equilibria in the quaternary (Na+, K+//SO42-, H2PO4--H2O) system at 298.15 K is studied by using the through translation and the combination of phase diagram analysis, respectively. The analysis of the phase diagram and water diagram of the quaternary system (Na+, K+// SO42-, H2PO4--H2O) would be used to design a process based on the extraction technology process of KH2PO4. The results demonstrate that the phase diagrams are valuable to guide the separation of salts during evaporating process. (C) 2016 Elsevier B.V. All rights reserved.