During the period of 1985-1986, the annual average of chlorophyll-a contents of sea water and phytoplankton cells in the estuarine area of the Changjiang River were 3.3mg/m3 and 3×107 cells/m3, respectively. The tendencies of the horizontal distribution and seasonal variation of the chlorophyll-a were very similar to those of the phytoplankton. In the flood season (from June to September) the phytoplankton patch of the largest range and the greatest density of cells occurred in the middle part of the investigated area and only a small patch whose cell density was one fifth of that of the flood season occurred near the estuary in the other months. In summer, the composition of the phytoplankton was very complex, the fresh-water species,Scenedesmus spp. and Pediatrum spp., speared in the estuary, the eurythermal and low saline species such as Skeletonema costatum(Grev.)Cleve occupied the mddle part and some tropical species, Hemidiscus hardmannicnus(Grev.) Mann and Amphisolenia bidentata Schroeder, etc, appeared in the southeastern and northeastern parts of the investigated area. The distribution patterns of the above phytoplankton species of different ecological natures showed the dynamic state of the different water masses in the investigated area. The main factor on the variation of phytoplankton cell numbers is the flow capacity of the Changjiang River. The phytoplankton peak(108 cells/m3) often occurred in the months with flow capacity of 34,000 to 40,000m3/s. When the flow capacity is more than 45,000m3/s, the transparency of the sea water falls to less than 0.5m, and becomes the limiting factor to the phytoplankton growth, although a great deal of nutrients were carried into the estuarine area by flood, yet those rich nutrients were not beneficial to the phytoplankton growth.