Abstract: Qingdong Sag is a typical marginal sag in Bohai Bay Basin, with characteristics of small area and shallow burial depth of source rock. The unclear understanding of hydrocarbon generation and evolution mechanisms, favorable hydrocarbon accumulation combinations and hydrocarbon accumulation pattern near source rock restricted the petroleum exploration in the sag. Based on analysis of regional tectonic settings, and combined with study on geochemical experiment, rock pyrolysis, thin section, fluid inclusion and thermal evolution, hydrocarbon accumulation conditions and exploration potential in Qingdong Sag has been re analyzed. The study results show that the upper sub-member of the fourth member and the lower sub-member of the third member of Shahejie Formation served as the main source rocks in Qingdong Sag,and the source rock deposited in saline lake basin in the fourth member of Shahejie Formation showed evolution characteristics of early mature and early hydrocarbon generation; Influenced by high heat flow in Tanlu Strike Slip Fault Zone, the depth threshold of hydrocarbon generation was shallower, and there were two hydrocarbon generation mechanisms of normal pyrolysis and immature-low mature at shallow burial depth;The regional oil and source rock correlation shows that crude oil in the northern fault horst zone was supplied by the mature oil in the northern sub-sag area and low mature oil in the middle sub-sag; During the deposition period from the fourth member to the third member of Shahejie Formation, the sediments supply continuously increased, and the deposition range of beach bar expanded and changed to bar sand deposits in the late stage; The reservoir quality in the third member of Shahejie Formation gradually improved upwards, and the thin and widespread sand bodies provided favorable pathway for hydrocarbon migration; Due to the structural inversion at the end of the Late Paleogene, the southern part of the northern fault horst zone uplifted from an early sub-sag, and local structural high points in the northern part changed to the southern part; Driven by the fluid potential in hydrocarbon accumulation period, oil and gas migrated from north and south directions towards structural high parts along faults and sand bodies in a stepped manner. The hydrocarbon accumulation pattern near source rock shows “hydrocarbon supply by two sub-sags, reservoir controlled by faults, and hydrocarbon migration and accumulation in a stepped manner” in fault horst zone in the northern Qingdong Sag, which is typical in the marginal sag in Bohai Bay Basin, showing excellent exploration potential.