Abstract: Based on data analysis of bathymetry, subbottom profile and cores that collected in Nanri Waterway in June and July 2016, we discovered and confirmed a thick muddy wedge deposit in the waterway. Four seismic units, SU1, SU2, SU3 and SU4 were divided according to highresolution seismic profiles on the distribution and characteristics of the sedimentary body. It shows that the sedimentary body has developed in the deep water area in the center of Nanri Waterway. It is thick in the middle, thin on both sides, about 2 km wide from east to west, and more than 20 m in thickness. The sediment composition of the wedge is mainly silty sand in the northern area of Nanri waterway, and silt mixed sand in the south. The seismic profile shows that there is a large acoustic shielding area in the sedimentary body, which is presumed to be the development of shallow gas that decreases gradually from north to south before it disappears. The formation of the sedimentary body is closely related to the terrain and the sands from Mulan stream and coastal streams or gullies to the sea are the main material sources of the sedimentary body. With results of sedimentary environment study, we also discuss the four sedimentary stages developed since the Late Pleistocene of Nanri Waterway.