Abstract: Baiyun sag, located in the marginal deepwater area of northern south China sea, is an important area for deepwater oil and gas exploration in China. The gravity-seismic imaging is the important methods to analyze the characteristics of the hydrocarbon source rock distribution, large structural trap, oil and gas accumulation and reservoir formation conditions. In this paper, through the calculation of gravity and gravity gradient inversion, and the analysis of the structure model of the seismic wave velocity Vs and seismic section, the free space gravity anomaly, bouguer gravity anomaly, horizontal, vertical gradient and density inversion images, the structure image of v wave velocity, high resolution seismic exploration profile image of the study area were achieved. The image of gravity data reveals that the strong negative abnormal in the center of Baiyun sag is associated with thicker deposition, basal ups and downs; the east of the sag is relatively high, related to Mesozoic residual strata; the north of the sag is abnormal as bandings, related to the of the concave-down basement where the slope extends to the oceanic basin. The sag and the south sag exist obviously layered negative residual density or relatively low density area, being the most reservoirs to holding oil and gas. The image of seismic data reveals: thin neck belt, outer edge uplift belt, ocean-land transition zone, and so on from north to south of Baiyun sag. During the epicontinental strong stretching thinning crust, there formed the sag structure of large delta-lacustrine source rocks sedimentary environment, and sediment source mainly comes from the north. As the strong thinning crust thin neck, there was obvious in Baiyun sag, resulting that the shelf slope break belt migrated from south to north to form the reservoirs: shelf edge of delta, deep-water gravity current channel and deep-water fan, and so on. Based on the analysis of high resolution seismic data for seismic section, we can identify many gas migration pathways, showing that shallow gas in Baiyun sag originate from deep strata.