Abstract: Based on the investigations on phytoplankton in May (Spring),August (Summer),October (Autumn) of 2016 and February (Winter) of 2017 in Anhai Bay of Fujian Province,the community characteristics of phytoplankton were studied. The results showed that 139 species of phytoplankton which belonged to 68 genera of 6 phylum (Oscillariophyta,Euglenophyta,Chlorophyta,Chrysophyta,Pyrrophyta and Bacillariophyta) were identified,while Cyclotella sp.,Skeletonema costatum,Melosira sp.,Nitzschia sp.,Lauderia borealis,Gyrosigma fasciola,and Chaetoceros socialis were dominant species,especially Cyclotella sp. and Skeletonema costatum existed all over the year. The diatoms were the main group of phytoplankton and the ecotypes of phytoplankton were mostly eurytopic species and freshwater species. The average cell abundance were 22.26×10⁴ cells/L in Spring,6.23×10⁴cells/L in Summer,10.61×10⁴ cells/L in Autumn and 54.21×10⁴cells/L in Winter,with an average of 23.33×10⁴cells/L over the year and the diversity of phytoplankton was 2.64. There were great differences in the cell abundance,species and diversity index of phytoplankton between high-tide period and low-tide period. The drainage of sewage discharging from Jinjiang Economic Development Zone Sewage Treatment Plant had a great influence on the growth of phytoplankton Cyclotella sp. and the abundance of antifouling species such as Euglena sp. and Scenedesmus spp. The fresh water had a great influence on the phytoplankton in some regions. The changes of phytoplankton dominant species and diversity index showed that the phytoplankton community in Anhai Bay was unstable.