The stable-phase equilibrium of the quaternary system Na+//Cl-, NO3-, and SO42--H2O and its ternary subsystems at 333.15 K was measured by the isothermal dissolution method. Different from the simple phase equilibrium of the systems at 353.15 K, there is double salt generated in the Na+//NO3- and SO42--H2O system at 333.15 K. The double salt was determined to be NaNO3 center dot Na2SO4 center dot H2O by combining the wet residual analysis and X-ray diffraction test. Based on this characteristic, the quaternary Na+//Cl-, NO3-, and SO42--H2O system has two invariant points and five univariant curves, which generate four fields corresponding to the crystallization of Na2SO4, NaCl, NaNO3 and the double salt. The areas of the fields indicate that the single salts have the solubilities in the sequence of Na2SO4 < NaCI < NaNO3. Meanwhile, the crystallization region of the double salt is relatively small, which indicates that the double salt will disappear when increasing the temperature further. Related results provide the important basis for the fractional crystallization process