Miaoxi Sag which located in the southeastern Bozhong depression in Bohai Bay Basin, is a hot area for hydrocarbon exploration in Bohai Sea area and has recently been proved to be a potential petroliferous sag. Tanlu fault belt passing through the area has resulted in strong structural transformation since Cenozoic, which changed the original appearance of Miaoxi Sag in the sedimentary period of the third member of Shahejie Formation which is the major hydrocarbon generating interval. Therefore, it made it difficult to evaluate oil resources in this area. Employing the research thought for reformed basins, based on more than 70 seismic sections, several well loggings and fission track data, the original sedimentary extent of the Miaoxi sag during the sedimentary period of the third member of Shahejie Formation is restored in this paper. The study shows that Miaoxi Sag had been strongly reformed since the end of the Eocene epoch, nearly one quarter of the third member of Shahejie Formation were denudated on the east of the sag. The Northern Miaoxi Rise and the Southern Miaoxi Rise were synsedimentary faulted blocks, which played the role of the provenance area for Miaoxi Sag in the sedimentary period of the third member of Shahejie Formation. It was connected with Bodong Sag on the central and north of Miaoxi Sag, and bordered on Huanghekou and Laizhouwan Sag to the south of the sag. Considering the original sedimentary extent of the hydrocarbon source rock in the third member of Shahejie Formation and current exploration practices, the hydrocarbon potential resources in this sag should be enormours.