The phase equilibria of the quaternary system Na+//Cl-, NO3-, SO42--H2O and the subsystems Na+//Cl-, SO42--H2O, Na+//Cl-, NO3--H2O, Na+//NO3-, SO42--H2O at 353.15 K were studied by the isothermal dissolution method. There are one invariant point, three univariant curves, three crystallization fields of single salt, three cocrystallization fields for two salts, and one cocrystallization field for three salts in the quaternary system. Neither double salt nor solid solution is found in the system at 353.15 K. At the invariant point of Na+//Cl-, NO3-, SO42--H2O system, the composition of the solution is NaCl 7.00 wt %, Na2SO4 1.59 wt %, NaNO3 47.79 wt %. The order of the solubility in the mixing solution of the single salts is Na2SO4 < NaCl < NaNO3. Moreover, the solubilities of the systems at 353.15 K were calculated theoretically by using the Pitzer ion interaction model. The high ionic strength was found to be a critical factor, which influenced the calculation accuracy significantly. All data and results obtained in this paper are of great significance to the design and optimization of the fractional crystallization process of the high-saline wastewater in coal chemical industry.