The Qarhan salt lake is the depocentre of the Qaidam basin during the Quaternary, with a very thick sequence of lacustrine sediments. Continuous lacustrine sediments provide excellent archives of detailed paleo- environmental data regarding climatic change in the Qaidam basin and northern Tibetan Plateau. In this study, fossil ostracods from the drill core (ZK53630- 1) sediment were analyzed in conjunction with AMS 14C dating. A totalof 10 ostracod species belonging to 7 genera are identified. The ostracod fauna can be subdivided into two assemblages from bottom to top: Ilyocypris biplicata- Limnocythere inopinata and Ilyocypris bradyi- Ilyocypris sebeiensis. The ostracod fauna and associated micro- fossils suggest the existence of a large freshwater to oligohaline lake in the Qarhan area during the Late Pleistocene (MIS 3a, 34~28 ka), coevaled with a warm and humid climate, which was completely different from the present- day hyper- arid salt lake environment.