Baxian sag, controlled by the Niudong fault, is a Cenozoic half-with faulting in west and overlap in east. Based on continuous 3D seismic data, various methods including fault throw method, balanced cross-section technique are adopted in this paper to analyze the structural characteristics and Cenozoic evolutionary process. The results show that Niudong fault can be divided into three sections: In north, it shows a series of NE-NNE fault terrace zone dipping east; in central, it presents as listric fault; in south, it is characterized as brush structure. Baxian sag is divided into three sections accordingly. Niudong fault shows a sectional activity in Paleogene, and activity strength migrates from south to north with time. There are mainly three periods: In Ek-Es4 period, north-central segment present a stronger activity; in Es3-Es2 period, south-central and north-central segments became stronger; in Es1-Ed period, the whole fault has strong activity while the north has stronger activity. Baxian sag underwent rifting stage in Paleogene and depression stage in N-Q. And the rifting stage is subdivided into three sub-rift screens: Ek-Es4 period、Es3-Es2 period and Es1-Ed period. The evolution characteristics of Baxian sag had an appreciable effect on development and distribution of source rocks, reservoirs, traps.