The Xijiang 30 sub-sag is located in the southwest of the Huizhou sag. In the early exploration stage, due to the poor quality of seismic data and few deep formation drilled wells, geological research and oil and gas discovery in the study area are mainly concentrated in the shallow regions, while relatively little attention is paid to the deep Wenchang Formation, resulting in a lack of systematic source-to-sink system analysis and favorable sand body prediction, which restricts the exploration process in the area. Using the latest 3D seismic and drilling data, the sedimentary system of the Wenchang Formation in the Xijiang30 sub-sag steep slope zone was systematically studied under the guidance of the “source-to-sink” theory. The results show that the Mesozoic granite in the Huixi low uplift provides provenance for the Xijiang30 sub-sag, and there are 6 catchment units in the source area, corresponding to 6 sediment transport pathways. The catchment area, catchment height difference, transportation distance, valley type, width:depth ratio and other parameters are calculated quantitatively. Based on the analysis of seismic sedimentology and tectonic background, it is considered that a transformation of the fan-braided sedimentary system occurred in the steep slope belt of the Xijiang30 sub-sag. Fan delta deposits developed in the fourth member of the Wenchang Formation, and 5 fans can be identified. The third member of the Wenchang Formation developed braided river delta deposits, and three lobes can be identified. The migration of rift strength and the change of paleogeographical pattern caused by the Huizhou Movement are the main reasons for the transformation of the fan-to-braided sedimentary system in the steep slope belt of the Xijiang30 subsag. From correlation analysis of various source-to-sink elements, it is clear that the catchment area and transportation distance are the main controlling factors of the scale of sand bodies in the steep slope zone of the Xijiang30 sub-sag, and it is pointed out that F catchment unit has the strongest sand transportation capacity. Its corresponding fan bodies 4 of Wen-4 member and lobes 3 of Wen-3 member have the largest distribution scale and the best seismic facies characteristics, which are conducive to the development of high-quality reservoirs, and it is a favorable zone for future exploration. The above analysis points out the favorable direction for the exploration of the Wenchang Formation in the steep slope belt of the Xijiang30 sub-sag, and provides a unique case for the study of the steep slope type source-sink system.