Sandbar is the main geomorphic unit in estuary, and due to the effects of runoff, tide and fresh-salt water mixing, the sediment movement there differs from others. Taking the Yangtze Estuary as an example, firstly, its sediment transport forms many circles both in longitudinal sections and transverse sections. Secondly, its sediment trapping is controlled by two different mechanisms, tidal current and salt water. The former contains the delay effect of sediment deposition and incipient motion, the asymmetry of flood and ebb, and the separation of flood and ebb, whereas the latter includes the influence of the saltwater wedge density current, the aggregation of the saltwater intrusion fronts, and the trapping of flocculation. Finally, its sediment re-suspension occurs three or four times ,not noly at the moment of flood or ebb maximumbut also at the transferring period from flood to ebb or from ebb to flood.