Abstract：Based on field geological survey and an analysis of such environmental indicators as grain sizes, magnetic susceptibility and carbonate content as well as ESR and U-series ages, this paper discusses the variation of the depositional environment in the Bangrong Formation during late Pleistocene. The analytical results show that the Peiku Co paleolake underwent a whole process of lake evolution：the initial shallow lake period and the development of the deep lake and the shrinkage into the shallow lake. During 127~15 ka B.P. in Late Pleistocene, the basin evolved into the strong rift stage. Being warm and humid,the water table of the lake rose, and the paleolake evolved into the shallow lake stage during 127~56 ka B.P. The paleolake went into the deep lake stage with the surface of paleolake wave rising during 56~31 ka B.P. The paleolake shrank into the shallow lake stage during 31~15 ka B.P. The emergence of freeze-thaw fold may suggest that this region went into the ice-border period.