Abstract: Xijiang (West River) is the largest river in the south of China, and Modaomen, the main river-mouth of Xijiang is the largest one of the eight distributary mouths in the delta of Zhujiang (Pearl River), It is an estuary.The estuary sediment is mainly terrigenous detritus carried down from xi-jiang. The spatial distribution of the sediment is controlled by the separation of flow of the run-off and the tidal current, and influenced by the cross-section gradient of the water surface.So the coarser sediment is at the eastern part and the finer one at the western part, together with intersection of the coarser and the finer sediments.Due to affection of the landform of the river-mouth bar, the fundamental characteristic of sediment distribution and grain-size probability curves along the bed of the subaqueous river channel exhibits an inverted-order from up-stream to down-stream.The genesis of transverse formation of the river-mouth bar is wave action, especially that in the south-west direction. In the mid 19th century, the deep bed of the subaqueous channel outlet deflects eastward at the southern side of Shilanzhou Island and the ebb-tide current, the residual current and the south-west nearshore current are all reversed in direction, and thus the embryonic form of the transvers river mouth bar is formed.In the mid 20th century the subaqueous channel outlet in the bar-crest is closed, and became a bow-shaped snoal. At present, the river-mouth bar continuously grows higher and greater.The shape of the grain-size probability distribution of the river-mouth bar sand at the bar-crest is concave downward very similar to the probability curve of the river-mouth bar sands of the modern Changjiang (Yangtze river) and the ancient river (early Tertiary period) of northern Jiangsu and western Liaohe. It is called transitional mixed texture. It's the sediment of the effective wave and flow in depositional process of sediment transports along the flowing direction. And it differs from the grain-size probability curve of river mouth bar sand with double saltation population concave upward, The latter is a reflection of the depositional process of bidirectional transport by swash and backwash.