Abstract: In order to establish the relationship between hydrocarbon characteristic variations and dynamic environments, the differences of hydrocarbon geochemical characteristics of reservoirs in various pressure systems (overpressure zone, transitional pressure zone and normal pressure zone) were studied. The Dongying and Zhanhua sags in the Jiyang Depression of the Bohai Bay Basin were taken as examples in this paper, the variations of sterane isomerization parameter, sterane light/heavy ratio with depth were revealed. The transition from overpressure charging to normal pressure driving was clarified. The typical profiles of the Boxing (single pressure structure) and Bonan (composite pressure structure) sub-sags in the Dongying and Zhanhua sags, respectively, were selected for the differential analyses of geochemical parameters and corresponding dynamic conditions in various belts. The variations of the geochemical parameters showed high correlations to overpressure driving and buoyancy driving. The development ranges of abnormal pressure during reservoir formation were studied as well, which are much larger than the present abnormal pressure distribution ranges. Especially in the step-fault zone belt, historic overpressure had greatly extended outwards. According to the geochemical characterization of dynamic conditions, the reservoir formation systems and hydrocarbon supply ranges were described in detail.