A lake core, located at 100 m to the southeast of the Tianshuihai Lake (35°21'N, 79°25'E, 4 840 m asl) on the southern flank of the west Kunlun Mountains, was drilled in July of 1995. The analytical results showed that there appeared rich ostracode fossils within the depth of 56.32 m in the sediment core. A total of 10 OwStracode species from the deposits were identified. Based upon the distribution of ostracode fossils within the strata, five ostracode assemblages were distinguished as follows:1. Leucocythere sp. -Leucocythere mirabilis assemblage. This assemblage, which consisted of Leucocythere mirahilis, Leucocythere sp., Leucocythere gongheensis, Candoniella lactea and Ilyocypris bipiicata, mainly indicated a fresh water lake environment with deeper and relatively stable water. It appeared in the middle and the lower parts (depth of 44.55-44.76m, 24.65-29.33m, and 8.54-8.85m) of the core. 2. Dolerocypris fasciata-Leucocythere mirabilis assemblage. It contained Dolerocypris fasciata, Leucocythere mirabilis, Ilyocypris biplicata, Leucocythere sp. and Candona sp. This assemblage indicated a fresh water lake environment with warmer temperatures. It occurred in the sediments with the depth of 5.48-5.78 m. 3. Eucypris inflata-Cyprideis torosa assemblage. The major components were euryhaline and brackish water species, such as Eucypris inflata, Cyprideis torosa. Appearance of these os-tracodes indicated that the salinity of water is above 5‰, and the ancient Tianshuihai Lake was actually a NaCl dominated inland saline lake. This assemblage occurred in the sediments with the depth of 0-2m. 4. Limnocythere dubiosa dominated assemblage. This assmblage represented a fresh water lake or oligohaline water lake with rich HC0₃^- in water. It appeared in the sediments with the depth of 2.46-2.76m. 5. Leucocythere mirabilis dominated assemblage, This assemblage, which nearly contained the single species Leucocythere mirabilis, indicated fresh water or oligosaline water lake. It occurred in the strata with the depth of 2.22-2.46 m. Based upon the distribution of ostracode fossils and their assemblage characters, sedimentary characters, ¹⁴C and U/Th dates, the environmental evolutions of the Tianshuihai Lake area since 240 ka BP had been preliminorily recognized. The evolution of the ancient Tianshuihai Lake can be divided into six stages. Inthe former four stages (242-59 ka BP, depth of 56.32-13.61m), this area was dominated by fresh water lake. During these stages of the evolution, by the influence of climatic changes, fluvial environment occurred many times in this area. In the fifth stage (59-25 ka BP, depth of 13.61-2.76m), this area was fresh water lake with warmer water temperatures. The sixth stage (25-17 ka BP, depth of 2.76-0 m) was in the period with strong climatic changes. During the earlier stage of last glaciation period, this area was fresh water lake or oligohaline water lake, reflecting a humid climate in that time. During the last glacial maximum, this area was brackish water lake even saline lake. It can be seen that the climate turned into drought from then on, and the ancient water level dropped rapidly.