Through systematic sampling and analyses of the lacustrine carbonate rocks in the Cenozoic continental strata in the northern QinghaiTibetan Plateau, by using the carbon and oxygen isotopes as proxies, the environmental evolution of the Early Cenozoic is revealed according to the palaeolacustrine evolu￣tion. The results indicate that the δ¹⁸O and δ¹³C in the bioclastic limestone samples have the lowest values in all the samples in this area, reflecting that they were deposited in an open freshwater lacustrine environment with a short period of water residence. There is a positive relation between the δ¹⁸O and δ¹³C in the micritic limestone samples, reflecting that they were deposited in an obvious closed saline water laustrine environment. The obviously enriching of δ¹³C in stromatolitic limestones may be related to the microbial activities. The δ¹⁸O value of the lacustrine carbonate rocks gradually increased from Eocene to Oligocene and then to Miocene. The δ¹³C changed from a negative value in the Paleogene to a positive value in the Neogene, which reflects that the lacustrine system in this area evolved from an open system to a closed one. Thus the prediction can be made that the palaeoclimate changed greatly throughout the PaleogeneNeogene period.