In order to effectively utilize the boron and lithium in the old brine and frozen brine of West Taijinar Salt Lake，the isothermal evaporation of potassium?extracted and frozen potassium?extracted brine in West Taijinar Salt Lake was conducted.The chemical analysis and x-ray powder crystal diffraction methods were used to analysis the content and phase compositions of solid phase.The theoretical route of crystallization was analyzed using the metastable phase diagram of the quaternary system Li⁺，Mg²⁺∥Cl^-，SO₄^2-—H₂O at 25 ℃.The results indicated that the evaporation process of the potassium?extracted old brine mainly precipitated halite，carnallite，bischofite，starkeyite，lithium sulphate monohydrate.Lithium salt began to precipitate when Li⁺ was enriched to about 5.4 g/L，while there was still no obvious precipitation when boron was enriched to 46.73 g/L.High quality bischofite could be gained at the evaporation stage of old brine with pre?frozen，and the concentration of lithium ion was increased compared with the former，but the lithium sulphate monohydrate was still precipitated in final.The crystallization route was basically consistent with the metastable phase diagram law of the quaternary system Li⁺，Mg²⁺∥Cl^-，SO₄^2-—H₂O at 25 ℃.