Abstract: Based on the data collected from two surveys in Zhao’an Bay and its adjacent waters in summer and autumn 2015, community structure of phytoplankton was studied and its relationships with environmental factors were performed with redundancy analysis. A total of 80 phytoplankton species were recorded and they belongs to 2 phyla and 33 genera, with 71 species of diatom in 28 genera and 9 species of dinoflagellates in 5 genera. Thalassionema nitzschioides, Nitzschia pungens and Skeletonema costatum were dominant species in summer while Bellerochea horologicalis, Chaetoceros lorenzianus, Skeletonema costatum, Thalassionema nitzschioides, Coscinodiscus asteromphalus, Coscinodiscus jonesianus and Thalassiothrix frauenfeidii dominated in autumn. The phytoplankton densities ranged from 3.32×10⁴ cells/m³ to 1.27×10⁸ cells/m³, and the cell densities in summer were higher than that in autumn. The phytoplankton abundance gradually increased from the inner bay to the outer bay. The average Shannon-Weaver index was 1.77 and 2.87 in summer and autumn, respectively. Redundancy analysis showed that active phosphate, chemical oxygen demand, temperature, salinity and pH were main factors affecting the phytoplankton community.