In Dongpu Sag, the sediments of the Member 3 of shahejie Formation consisted of dark-colored mudstone of deep water facies interlayered with sandstone and salty-gypsum, and this kind of sandstone is the important oil-bearing reservoir in Dongpu Sag and its origin was explained to be deep water gravity flow sediments according to the static sedimentary model. During the process of exploration, more and more evidences are found to indicate that there are traction flow and exposure to air markers in the sandstone. So, it is needed to reunderstand its origin and sedimentary models. In Dongpu Sag, during the sedimentation of Member 3 of Shahejie Formation, the lake 1evel fluctuated frequently with long scale cycle and short scale cycle. According to the data of well log, response of resistivity and core, cycles of 5 scales were recognized, and the frequency of the fifth scale cycle is 1000 times/Ma. The minimum extent of lake level fluctuation is calculated according to the thickness of single halite bed or single gypsum bed, and the maximum water depth is 30 meters. Combining with the palaeotopography character, the reason of large transference of lake shoreline in landscape orientation has been explained. On the basis of the study, a dynamic sedimentary model is established, as it is revealed in this model, delta and beach sandbody was deposited along the shoreline and black mud with thin layer carbonate rock was deposited in the deposition center during the high stand time, and during the low stand time, the shoreline shifted about 10~30 km toward the basin center. The sand body early deposited along the shoreline was eroded and was prograded toward the sedimentation center of the basin. Because of the fluctuation and lake level and the shifting of shoreline, the paleogeograhy of highstand time and lowstand time are very different. According to this dynamic sedimentary model to predict the distribution of sand body, there will be more clear result in the petroleum exploration.