Totally 284 phytoplankton water samples were collected in a multidisciplinary investigation in East China Sea (24°00'-32°00'N,120°00'-128°00'E) from December 23^th 2009 to January 5^th 2010. Phytoplankton species composition and assemblage structure were studied by Utermhl method. A total of 171 taxa which belong to 67 genera of 4 phyla were identified. The phytoplankton community was mainly composed of Bacillariophyta and Dinophyta, and there were also a few species belonging to Cyanophyta and Chrysophyta. Most of the ecotypes of phytoplankton were temperate coastal species, and dominant species were Paralia sulcata, Thalassionema nitzschioides, Pseudo-nitzschia delicatissima, Thalassiosira angulata and Thalassiosira subtilis etc. Cell abundance ranged from 0.356×10³ to 142.578×10³cells/L with an average of 14.137×10³cells/L, of which Bacillariophyta was the major group ranging from 0.356×10³ to 142.578×10³cells/L with an average of 13.023×10³cells/L, followed by Dinophyta, ranging from 0.356×10³ to 11.378×10³cells/L with an average of 1.177×10³cells/L. Horizontally, the surface distribution of cell abundance in survey area was mainly dominated by Bacillariophyta and the high value appeared in the sea northeast of Changjiang estuary. Dinophyta mostly concentrated in the southern and southeast part of the survey area where Kuroshio water flowed by. Vertically, the maximum of cell abundance was found in subsurface water, and along with the increase of the depth cell abundance firstly increased then decreased. According to the Pearson correlation analysis, the distribution of phytoplankton cell abundance was positively correlated with phosphate and silicata, negatively correlated with temperature, and nitrate's effect on cell abundance wasn't obvious.