Abstract: Chexi sag is located in the south of Bohai Bay, it is a secondary tectonic unit of Jiyang depression. In this paper, the mech-anism of HC migration downward has been studied on the basis of the analyzing results about the geologic characteristic combined with oil-source correlation, the distribution of effective source rock, allocation of source, reservoir and cap rock, reservoir forming dynamic and conduit system. The study results indicate that the source of Es3 is effective source rock in Chexi Sag, and the oil from source of Es3 migrated downward to the reservoir of Es4. It is considered that the main HC accumulating conditions are mud seals over source rocks, oil and gas migration downwards into reservoir under the oil source by overpressure of the oil source beds, and the good transmission system under source rocks are the better is the physical characteristic of reservoirs, the more easy the oil formed. There is a positive correlation between the downward HC migration distance and the overpressure of the oil source bed. Physical characteristic of reservoirs controlled the HC existence of reservoirs. The above concepts improve knowledge of hydrocarbon accu-mulation theory of Jiyang depression. It can be used for guiding the HC exploration in Bohai Bay.