In view of the excessive content of water insoluble matter in the five?effect evaporation process of vacuum NaCl-Na₂SO₄ co?production，which leads to excessive brine turbidity and affects the quality of sodium sulfate，it is of great significance to study and analyze the source of water insoluble substance and find out the method to control the water insoluble substance.X-ray diffraction（XRD），scanning electron microscopy（SEM-EDS） and infrared spectroscopy（FT-IR） modern analysis techniques were used to analyze the phase composition and morphology characteristics of water insolubles in sodium sulfate.It was determined that the main component of water insoluble in sodium sulfate was calcite calcium carbonate.It was found that the formation of water insoluble matter was mainly due to the high calcium ion concentration in the salt system.At this time，the solution was in an unstable state with high supersaturation，rapid precipitation of calcium carbonate，large number of crystals and small particles.The small particle calcium carbonate floated in the circulating brine and entered the sodium sulfate system through brine transfer，which was added with the calcium carbonate produced by the sodium sulfate system itself，resulting in the content of insoluble matter in sodium sulfate exceeding the standard.Aiming at this problem，the scheme of source regulation was put forward.In the purification process，when the addition amount of sodium carbonate was adjusted from 1.32 g/L to 1.35~1.38 g/L，the balance between calcium and carbonate in brine could be controlled，and the calcium ion concentration in salt and sodium sulfate system could be reduced，so as to control the excessive content of water insoluble substances.In the new round of industrial production，using the method of source control，the water insoluble matter in sodium sulfate had been effectively controlled，and the quality of sodium sulfate had been significantly improved.