Xiao Yangshan is located on the north of Yangshan port, which is the first offshore deepwater port in China. Phytoplankton community structure of Xiao Yangshan sea area was investigated and analyzed from January to December in 2012. A total of 183 phytoplankton species, belonging to 7 phyla and 77 genera, were identified. Diatom, including 51 genera and 143 species which accounted for 78.14% of species richness, was the main phytoplankton group.The annual average cell abundance of phytoplankton was (7.01±6.26)×10⁴ cell/L. Skeletonema sp. and Paralia sulcata were the main dominant species through the year, while Thalassiosira leptopus, Nitzschia frustulum, Rhaphoneis rhomoides and Cyclotella meneghiniana were the common species. Based on analysis of diversity, the annual average values of Shannon-Wiener index, Margalef index and Pielous index were 2.59±0.75, 4.00±1.44 and 0.55±0.15, respectively. The average number of Diatoms Index was 1.06±0.20. Redundancy analysis (RDA) showed that salinity, water temperature, dissolved oxygen and suspended solids concentration were the main environmental factors with great influences on phytoplankton.