Abstract: The western slope of the Xihu Sag is one of the favorable areas for oil and gas exploration and development in the East China Sea. Understanding the accumulation mechanism and distribution pattern of lithologic bodies, particularly sand-rich deposits, are the keys to petroleum explorations in the western slope during the transition from tectonic traps to tectonic-lithologic composite traps. Linking sediment supply variations and tectonic evolutions in source-to-sink systems, we summarized the characteristics of the provenance, sand transport pathways, and accumulation system of the Pinghu Formation in the X-structure in the western slope. Our results show that the provenance of the X Structure is complex, with a predominant sediment influx from the Haijiao Uplift to the west and a minor contribution from the Baoyunting paleo-high. Three different types of canyons: V-shape, W-shape, and U-shape canyons, served as the major sand transport pathways in the study area. According to the variations of provenance, sand transport pathway, and sand accumulation system, the sandbody-controlling patterns in the study area can be classified into four types: gently sloping flexural source-to-sink system, gently sloping synthetic fault source-to-sink system, gently sloping antithetic fault source-sink system, and steeply sloping uplift with the axial source-to-sink system. The reverse fault root and BT ancient uplift surrounding the lower section of the Pinghu Formation in the central part of the X structure, the descending plate root of the forward fault terrace area in the middle section of the Pinghu Formation in the central and northern parts, and the low potential area controlled by the flexural landform in the upper section of the Pinghu Formation on the northwest side are favorable for sandstone development and the are key areas for the development of lithological traps and high-quality reservoirs.