Abstract: The eastern sag is one of the three rich-hydrocarbon sags in Liaohe basin. Based on the chemical dynamic theory and digital simulation of vitrinite reflectances, the paleotemperatural evolutions of 14 wells in the eastern sag are shown, and then the geothermal gradient of every period during cenozoicera is reconstructed. That reveals the total characteristics of paleotemperatural evolution in the eastern sag. The geothermal evolution is flexuous since Tertiary, that is high paleotemperature in the early Shahejie, low in the middle-late Shahejie and high again in the Dongying stage to Neogene. To total region, the geothermal gradients (35℃~40℃) are higher in the southern and northern part than that (29℃~35℃/km) in the central part.