Abstract: Based on quaternary phase diagram of Na<SUP>+</SUP>,Mg<SUP>2+</SUP>//Cl<SUP>-</SUP>,SO<SUP>2-</SUP><SUB>4</SUB>·H<SUB>2</SUB>O,magnesium sulfate monohydrate and sodium chloride were prepared by using mother liquid of sodium sulfate and magnesium chloride hexahydrate as raw materials.Optimized and optimium conditons were confirmed as follows:mother liquid of sodium sulfate was 10% overdosed,reaction temperature was 55 ℃,evaporation temperature was 100 ℃,and endpoint density of evaporating liquid was 1.326 g/cm<SUP>3</SUP>.Scale-up experiment was carried out on the basis of lab experiment.Results showed that magnesium sulfate monohydrate product was up to the top grade of magnesium sulfate monohydrate of Q/YHJ-03-022-2004 and soldium chloride product met GB 5462—1992.In this process,mother liquid of sodium sulfate and impurity magenium chloride that affact the quality of sodium sulfate product were both effectively utilized,and the problem of low yield of magnesium sulfate monohydrate due to limited natural production of salt pond in sodium sulfate companies was resolved as well.&nbsp; 