By studying the vertical distribution characteristics of brackish fossils in the Paleogene and using the paleontological-lithofacies synthetical analysis based on the new stratigraphic classification, the author has drawn the Paleogene water-level change curve of the Bohai Bay Basin, and found that the brackish biota appeared in the early stage of water transgression and disappeared in the stage of the greatest water transgression. The author has also foimd that there are no immediate relations between the water-level change curve in the Bohai Bay Basin and the global sea-level eustatic curve, that is, the rise of water level was not influenced by the rise of sea level. All these characteristics show that there was no sea transgression in the Bohai Bay Basin during the Paleogene. The non-sea-transgression theory has important significance for petroleum exploration in the Bohai Bay Basin.