Abstract: In order to fully use magnesium resource in salt lake of Qinghai，under the guidance of the equilibrium phase diagram of the system Na+， K+， Mg2+∥Cl- －H2O at 25℃，studies on evaporation process in the purification of bittern with high magnesium content from Cha Erhan salt lake were carried out and the crystallization law of impurities in the process of evaporation was also studied.Taking 1 L high-magnesium brine mother liquor as a standard，when the evaporated water was 307 g，the content of magnesium chloride reached 33.697%（mass fraction），the content of sodium chloride reduced to 0.007% （mass fraction），the content of potassium chloride reduced to 0.134%（mass fraction） and the content of magnesium increased by 3.412%.Decreasing rates of sodium content and potassium content were 91.86% and 19.28%，respectively.Results showed that the process of evaporation crystallization increased the concentration of magnesium chloride，separated Na+，K+，and Ca2+，purified bittern with high magnesium content，and provided experimental basis for preparing series high  value added magnesium products. 

Key words: bittern with high magnesium content, evaporation crystallization, phase diagram, purification