【Objective】The present paper aimed to investigate the characteristics and effective factors of phytoplankton s community structure in Dongji sea area of Zhoushan.【Method】 Based on the continuous survey in four seasons on phytoplankton which were carried out from February,2017 to October,2017.The species composition,dominant species,abundance of phytoplankton,the diversity index and their relationships with environmental factors were analyzed.【Result】A total of 142 phytoplankton species were identified,which belong to 8 phyla,70Taxa,including Bacillariophyta (47 genera and 104 species),Pyrrophyta (12 genera and 27 species),Cryptophyta (4 genera and 4 species),Chrysophyta and Cyanophyta (2 genera and 2 species),Chlorophyta,Xanthophyta and Euglenophyta (1 genera and 1 species) were found.Bacillariophyta and Pyrrophyta were the main phytoplankton,and there were totally 20 kinds of dominant species.The mean abundance of phytoplankton was 0.46*10~3 cells,abundance in summer and winter was obviously higher than in spring and autumn.The mean of phytoplankton' s Shannon-wiener diversity index was 3.07,the mean of Margalef richness index was 4.59,and the mean of Pielou evenness index was 0.65.Three indexes show that the changing tendency was higher in autumn and winter and lower in spring and summer.【Conclusion】RDA analysis analysis indicated that the distributions of phytoplankton were related to water temperate,salinity,dissolved oxygen and nutrient.In addition,it may be affected by geographical location,climate change and anthropological activity.