Abstract: According to the petrology, granularity distribution, sedimentary structure and well logging features of the first member of Dainan formation in the western inner slope of Qintong Sag, it is considered that there are 6 types of sedimentary microfacies including underwater distributary channel, interdistributary bay,channel mouth bar, distal bar, sand sheet and predelta mud in the study area. Underwater distributary channels are distributed from southeast to northwest, and the sheet sand is widely distributed. As a whole, the first member of Dainan formation is a positive cycle of sedimentary characteristics, from Ⅲ oil toⅠoil group, the water area is increasing, reflecting a deepening water falling, sedimentary thickness of retrogradation process.From bottom to top, the sedimentary facies sequence gradually evolved from delta front subfacies to predelta subfacies.