The metastable phase equilibria of the quaternary system NaCl＋Na2CO3＋Na2SO4＋H2O were studied at 273.15 K. The salts＇ solubilities, densities and pH values of the equilibrated solution in this system were determined. According to the experimental data, the metastable equilibrium phase diagram, the diagram of density vs. composition and pH vs. composition diagram were plotted. The phase diagram consists of five univariant curves, four crystallization fields and two invariant points. The four crystallization fields correspond to sodium carbonate decahydrate （Na2CO3·10H2O）, sodium sulfate decahydrate（Na2SOn-10H2O）, sodium chloride（NaCl） and burkeite（2Na2SO4. Na2CO3）, respectively. The crystallization field of sodium sulfate decahydrate（Na2SO4.10H2O） is the largest, which indicates that sodium sulfate is easy to saturate and crystallize from solution at 273.15 K.
