Abstract: This study restored the balanced cross sections of the Tabei sag and calculated the extension amounts and extension rates of the sag using three typical seismic profiles selected from the western Tabei sag,Erlian Basin using the 2D Move software.Then,it analyzed the geological and tectonic evolution characteristics of the Tabei sag since the Early Cretaceous.The results indicate that:(1)Due to the effects of the main normal fault layer on the northwest side,the whole Tabei sag has evolved into a single-faulted dustpan-shaped faulted lake basin that is faulted in the west and is overlapped in the east.Moreover,it consists of a fault in the lower part and a depression in the upper part vertically.(2)The variation characteristics of the extension amounts and extension rates in each period of the Tabei sag match the tectonic evolution process of the sag,and all of them show a strong to weak trend,indicating that the extension in the Tabei sag is in a weak-strong-weak trend.(3)The Tabei sag mainly underwent four tectonic movements:①During the sedimentary period of the Aershan Formation,the faults in the Tabei sag began to develop,the sag subsided under the control of normal fault activities, and the Tabei sag began to take shape.②During the sedimentary period of the Tenggeer formation,the faults inside the sag continued to develop,and the whole sag was significantly controlled by the fault depression.The extension amount and extension rate gradually increased and reached their peaks during the upper Tenggeer formation.③During the sedimentary period of the Saihan formation,the Tabei sag was slightly affected by faults overall,and the sedimentary center was gradually far away from the main fault. During this period, the Tabei sag was characterized by depression,subsidence,and overlap unconformity.④Since the end of the Saihan formation,the fault depression activities have disappeared,the basin has gradually contracted, and the terrain has tended to be flat.Since then,the Tabei sag has been continuously uplifting.