Abstract: By analyzing the drilling data and seismic data of Cangdong Sag and its surrounding area, the spatial distribution and current structural feature of residual strata in the Kongdian Formation, Cangdong Sag is obtained. Combined with the sedimentary characteristics indicated by 1 120 m core from 65 wells, logging information from over 340 wells, mud logging information from over 600 wells, 32 seismic lines and other data, it is preliminarily considered that the lake basin in Kongdian Formation, Cangdong Sag overall experienced 4 stages in the depositional stage, i.e., early depression period(sedimentary period of the 3rd Member of Kongdian Formation), depression period(sedimentary period of the 2rd Member of Kongdian Formation), later depression period(sedimentary period of the lower sub-member of the 1st Member of Kongdian Formation) and faulted period(sedimentary period of the upper sub-member of the 1 st Member of Kongdian Formation). The 4 stages of paleogeography evolution in the lake basin is as below:alluvial fan-alluvial plain(or braided river delta) of the 3 rd Member of Kongdian Formation-shore-shallow lake sedimentary system→braided river delta-deep lake sedimentary system of the 2nd Member of Kongdian Formation→alluvial fan-alluvial plain-shore-shallow lake sedimentary system of the lower sub-member of the 1st Member of Kongdian Formation→alluvial fan-alluvial plain-gypsic saline lake sedimentary system of the upper sub-member of the 1st Member of Kongdian Formation.