Abstract: Based on the study on Ostracoda collected from a total of six drilling holes in the Charhan lake area,Qinghai Province,four ostracods assemblages indicative respectively of Saline,brackish, fresh-water and swamp facies have been recognized.It was found that since 0.78 Ma B P the area was characterized by a dominance of shallow lake and littoral environment during the long duration of cold,dry climate.The appearance of peat sediments also indicates the Presence of littoral zone at the southeast part of the lake.It was only at the beginning of the last Great Glacial Stage(c.24 ka B P) that the salt lake was abruptly formed.At the west part of the Qaidam Basin the salt deposits have been formed since Late Pliocene.But the lake had undergone some freshening as a result of climatic changes.The extinetion of fresh-water or brackish lake was related to the sudden decrease in rainfall and runoff resulting in little or no solution of the salt deposits even in the locene times.So the formation of the Charhan salt lake is thought to be the result of the Earth surface system megaevolution.