Huanghekou Sag is located in the northeast of Jiyang Depression in Bohai Bay Basin. Due to the lack of unified 3D seismic data and poor quality of regional mid-deep seismic imaging in 3D seismic data，it is difficult to carry out the whole area correlation and results in many problems，such as unclear recognition of mid-deep sag structure，inconsistent understanding of the division of the Paleogene strata in the whole area，serious difference of the strata thickness and their distribution regularity of Kongdian Formation-the fourth member of Shahejie Formation（Ek-Es4）with the surrounding area.In the process of continuous exploration in mid-shallow strata，problems such as the mismatch between the hydrocarbon resources and the reported reserves and the lack of new exploration strata have severely restricted the exploration process. To this end，the new three-dimensional contiguous seismic data are used and the lithology，paleontology，zircon，isotopic dating and other data of new drilled wells in the study area，etc. are integrated to study the regional stratigraphic correlation and evolution of Paleogene. The attribution of unconformities at all levels are defined，the interface between Es3 and Ek-Es4 is adjusted，and the Cenozoic bottom surface is recognized. After the adjustment，the bottom surface of the Es3 has been move downward by 900 m，the bottom surface of the Cenozoic has been move downward by 2 300 m，and new exploration strata of Ek-Es4 has been found. According to the new understanding of the stratigraphic structure，Huanghekou Sag is defined with a dustpan style and the distribution characteristics of Ek-Es4 has been clarified. The scale of hydrocarbon resources in the study area is recalculated with an increase of 740 million tons，and the contradiction between resource and reserves reported has been solved. The mid-deep strata with a number of favorable traps in Kongdian Formation has great exploration potential.