Abstract: <P class=MsoNormal><SPAN lang=EN-US>Solid-liquid phase equal relationship of ternary water-salt system of NaCl-(NH<SUB>4</SUB>)<SUB>2</SUB>SO<SUB>4</SUB>-H<SUB>2</SUB>O </SPAN><SPAN lang=EN-US>was studied at 298 K.The solubility data of ammonium sulfate and sodium chloride in water were determined and phase diagram was drawn.Research results showed that the phase diagram of this system mainly had six regions,namely,pure ammonium sulfate crystal region,pure sodium chloride crystal region,ammonium sulfate dominant mixed crystal region,sodium chloride dominant mixed crystal region,ammonium sulfate dominant and sodium chloride dominant cocrystallization region,and unsaturated solution region.But because of the corelation of ions,it still exists some cross regions in phase diagram,which need the further study.<BR></SPAN></P>