Abstract: Through a fine interpretation of the high-quality 3D seismic data, i.e. broadband, wide-azimuth and high-density, acquisitioned in recent years, this paper finds that a series of small faults are developed in Mesozoic strata in the ywb area region of the southern Ordos Basin. Based on the research on the tectonic structure and evolution of the fault, this paper analyzes the system of the fault, its activity period and its dynamic mechanism. It is suggested that the small fault played an important role in connecting sources rocks with reservoir beds, controlling oil/gas migration and accumulation. The cracks associated with small fault can improve the physical properties of tight sandstone reservoir. The results of the study have a new view on the main controlling factors and the law of hydrocarbon accumulation, and also have an important guiding significance for oil/gas exploration in this area.