Abstract: To achieve the optimal condition for desulfurization by freezing，the freezing experiment on Yiliping salt lake brine was conducted under different densities of 1.215，1.218，and 1.221 g/cm3 and temperature of -5 ℃，-10 ℃，-15 ℃，and -20 ℃，respectively.Results showed that the density of 1.215 g/cm3 and temperature of -15 ℃ can be selected as the best conditions for desulfurization.When the concentration of SO42- was decreased to approximately 1/3 of raw brine，the separation of solid and liquid was completed.The simulated natural evaporation on residual liquid has been conducted in laboratory.As far as the characteristic coefficient of brine was concerned，the brine frozen still belonged to magnesium sulfate subtype.Ho- wever，the evaporation behavior of liquid point could be in conformity with the Na+，K+，Mg2+∥Cl--H2O quaternary system at 25 ℃ and that of raw brine untreated should be explained by the Na+，K+，Mg2+∥Cl-，SO42--H2O quinary meta-stable system，which means that the chemical type of magnesium sulfate sub-type brine can be converted from quinary to quaternary by freezing in terms of application of phase diagram.It will provide technical reference for the exploitation of Yiliping salt lake and theoretical guidance for conversion of sulfate type salt lake brine.

Key words: Yiliping salt lake, magnesium sulfate subtype, quinary system, quaternary system, conversion of brine