In situ data show that fluid mud of the Changjiang Estuary consists of fine sediment ranging from 8 to 11.5 IJm (median grain-size) including 28.8%-36.4% of clay. The composition of the clay is illite, chlorite, kaolinite and montmoillonite. The FM is a layer of high sediment concen-tration near the bed and results from flocculation under the environment of salt and fresh water mixing. Three kinds of FM have been identified under typical dynamic conditions: the first one is formed at slack water of ebb tide during the flood season, with the characteristics of extended area and low thickness; the second one is formed following a storm, characterized by large area and larger thickness; the third one is formed around the front of the saltwater wedge, characterized by small area but large thickness. In the dredged channel, the FM can be accumulated up to 1 m thick In general, FM will change with the alternation from spring to neap tides, flood and dry seasons. Drastic change can happen during storms. At the same time, the change of FM is closely related to the erosion and growth of the mouth bar.