Lucaogou formation in Jimsar sag is important lacustrine tight oil bearing series in Junggar basin. To get more understanding on environmental characteristics of the paleolake, experiments are carried out by means of phosphoric acid method on carbonate in Lucaogou formation in Jimsar sag. Results indicate that δ13CPDB of carbonate range from 6.8‰ to 9.7‰ with average of 8.3‰, and δ18OPDB range from -11.9‰ to -4.3‰ with average of -6.2‰. Character and variation of δ18OPDB and δ13CPDB indicated that the paleolake during early stage was of open lake with short periodic fresh water injection, while paleolake during late stage was of highly confined saline lake with long hydraulic residence time. Massive nourishment come up together with magmatic activities, organisms flourished, and δ13CPDB exhibited heavier than usual as a result. Precipitation which is rich in light oxygen isotope induce cyclic fluctuation in δ18OPDB. During sedimentation of Lucaogou formation, the paleolake is featured by high salinity with frequent variation. Influenced by cyclic precipitation and intensive evaporation, water level of paleolake varied in process of high-low-high, or high-low, and salinity also varied low-high-low, or low-high.