The sodium dihydrogen phosphate is one important chemical product, which has the widespread application in medicine, food, chemical industry, water treatment, agriculture aspects and so on. The solidliquid phase equilibrium and physic-chemical properties of the quaternary system Na+//H2PO4-, Cl-, SO42--H2O at 298.15 K were obtained by means of an isothermal solution method. There are four crystallization regions, one invariant point, and four univariant curves in the system. The crystallization region of NaH2PO4 center dot 2H(2)O (II) is the smallest, and its solubility is the biggest, which indicated that NaH2PO4 center dot 2H(2)O has a strong salt-out effect on Na(2)SO(4 center dot)10H(2)O, Na2SO4 and NaCl. The physic-chemical properties (refractive index, conductivity, pH values, density and viscosity) of solution changed regularly with the increasing mass fraction of NaH2PO4. All results obtained in this experiment can be used for the solvent extraction process of NaH2PO4 in the industrial production and further theoretical studies. (C) 2015 Elsevier B.V. All rights reserved.