Abstract: The Sansha Canyon, as a path for sediment transportation, starts from the shallow waters platform between the Yongxing island and the East island. It brings a large amount of carbonate clastics down to the northwest sub-basin of the South China Sea. It is the largest carbonate type canyon found so far in the South China Sea in typical V and U shape. In the cross section, the canyon can be divided into three parts, outer side, slope and valley. Both the outer sides and valley bottom are rather gentle, while the slopes are quite steep. The canyon can also be divided into three parts along the pathway, i.e. the upstream, midstream and downstream. The upstream part is steep with high varying slope, while the downstream is opposite. A large amount of mass deposits coming from submarine slides around the atolls are the source area of clastic sediments. On other hand, the midstream and downstream are characterized by gentle bottoms and the deposition of clastics coming from both source area and canyon slopes and finally settle down in a trumpet shape in the northwest sub-basin. The study of Sansha Canyon is helpful to the understanding of sediment delivery mechanism and the formation of deep water carbonate reservoirs between the coral reef platform and a deep sea basin.