Focusing on the northwestern part of Yanan sag of Qiongdongnan basin, we studied the sedimentary characteristics of Yacheng formation in Oligocene mainly through composition, grain size and sedimentary structure analysis. Typical braided delta sedimentary system has been identified in Yacheng formation. The lithology of the braided delta system is characterized by coarse clastic rocks which contain several thin coal seams and marine authigenic minerals such as glauconite. The rocks have multiple grain size cumulative curves both characterized by tractive and gravity flow. Sedimentary structures include multiple beddings such as parallel, tidal, uniform beddings and climbing-ripple cross-lamination, and many erosion surfaces and deformations, reflecting the complex and changeable hydrodynamic condition. Assisted by the logging curve characteristics and previous works, we constructed the sedimentary model of the braided delta system. We suggest that the braided delta system of the Yacheng formation in the study area was formed in the setting of balance filling in the early forming stage of the basin and developed near the vanish end of the main fault of the half graben. The braided delta system mainly includes subfacies of braided delta plain and braided delta front, and the main source areas are the Hainan uplift and Yacheng heave. Coal seams of the braided delta system are high quality source rock, together with the coarse grain rocks, forming the favorable lithological assemblage, which is important to the formation of the Yacheng13-1 gas field.