<p>The Funing Formation in the Gaoyou Sag of the Subei Basin is the key horizon of oil and gas exploration, but the understanding of its sedimentary facies is still ambiguous, which restricts the extraction of resources in the Subei Basin. To overcome this challenge, based on core observation, logging and 3D seismic data, the sedimentary facies of the Funing Formation in Gaoyou Sag were reassessed, the sedimentary evolutional model was recreated, laws governing of the distribution of sand bodies was redefined, and its oil and gas geological significance was delineated. Based on the results, the northern slope area of Gaoyou Sag has had shallow water depth, sufficient source supply, gentle slope, hot and humid climate, shallow water depth and frequent changes during the deposition of the first members of the Funing Formation, all of which provided favorable geological conditions for the development of shallow water delta. Furthermore, shallow water delta facies are developed in the study area, which is characterized by typical delta front traction flow, strong hydrodynamic force and frequent change of water depth. The vertical rhythm is characterized by a discontinuous positive combination, where the estuary dam is not developed, which shows that the diversion of the distributary channel is frequent. Moreover, the sand body morphology of the first member of the Funing Formation shows the typical top flat bottom convex characteristics due to multi-stage channelling, extensive development of sand bodies in underwater distributary channels. Finally, the Funing Formation in the study area is mainly a sand body of traction flow origin which is distributed in a large area, which takes the form of a shallow water delta deeper into the basin.</p>