Abstract: Xihu Sag is a hydrocarbon-rich sag in the East China Sea Shelf Basin. Well B1, drilled in 2001, revealed the existence of Baoshi Formation in Xihu Sag for the first time. Baoshi Formation, deposited in early-middle Eocene, is the product of strongly fault depression period. Due to the deep burial and the scarcity of drilling data, the understanding of the sedimentary facies distribution and sedimentary characteristics is still at the starting stage. Based on the seismic data and information of drilling wells, this paper discusses the distribution pattern of sedimentary facies in Baoshi sedimentation period, confirms it was generally belonged to the semi-closed shallow sea environment. It is suggested that delta, fan-delta and tidal flat deposits were developed along the west flank of the sag, small-scaled fan-delta deposits along the east flank, and the deposits of semi-closed shallow sea in the middle of the sag. It provides geological basis for the exploration of oil and gas in the frontier of Xihu sag.