Abstract: Species composition and cell abundance of phytoplankton, and its relations with environmental factors were investigated in the Changjiang River estuary of east China sea (30°50′～31°50′ N,121°50′～123°00′E). Atotal of 110 taxa belonging to 45 genera of phytoplankton in the investigation area were identified. The main species of phytoplankton resulted in red tide were Skeletonema costatum and Prorocentrum dentatum, etc. Phytoplankton abundance in the day was higher than that at night. The vertical distribution of phytoplankton in different water bodies was not obvious. The ecological groups of phytoplankton were freshwater group, estuarine and brackish water and low salinity group in the coast, high salinity and warm water group in off-sea, and widely ranging group. The abundance of phytoplankton in the Changjiang River estuary was 1.6×10³～75.2×10³ind·dm^-3. The species composition and cell abundance of phytoplankton were related to the freshwater pouring into the Changjiang River estuary. The three waters in the area affecting the distribution of phytoplankton were the Changjiang River estuary water, freshwater, and warm water from outsea.