The large-scale oil pools are developed in Yanchang formation in Ordos Basin. The research on the hydrodynamic system will be of service to the future accumulation study and exploration. First, through the study of acoustic travel time of logging, the low compaction of mudstone at the stage of maximum burial depth is acquired; then, the overpressure is calculated. Last, according to lithology, physical property and pressure distribution, the hydrodynamic system is analyzed. The Ch6-Ch8 reservoir group is a super pressure hydrodynamic system under the action of unstable compaction of mudstone at the major pool forming period.