Abstract: 【Objective】 To clarify the composition of phytoplankton community and its correlation with environmental factors in Haiwei Bay, Hainan, so as to provide reference for the protection of phytoplankton diversity and water environ-ment in Haiwei Bay.【Method】In July(summer) and September(autumn) 2019 and July(summer) 2020, water samples were collected from 12 fixed sampling points in the coastal waters of Haiwei Bay, Hainan Island for 3 cruises. After returning, phytoplankton community structure, water quality and surrounding environment were identified and recorded, and the correlation between phytoplankton community structure and environmental factors was studied through redundancy analysis(RDA).【Result】147 species of 4 phyla of phytoplankton were found in summer cruises from 2019 to 2020, among them Bacillariophyta species(121 varieties) accounted for the most, followed by Pyrrophyta species(18 varieties). The dominant algae species in the waters were Chaetoceros curvisetus, Thalassionema nitzschioide, Chaetoceros decipiens, Thalassiothrix frauenfeldii, Bacillaria paradoxa, Asterionella glaciali and Chaetoceros pseudocurvisetus, etc.;in the investigated sites, the average cell density was 17.50×10⁴ cells/m³, Shannon-Wiener diversity index(H') was 3.07-4.33, the Pielou evenness index(J') was 0.58-0.85, and the richness index(D) was 1.34-5.12. 86 species of 3 phyla of phytoplankton were found in autumn cruises in 2019, and the dominant algae species in the waters were C. curvisetus, C. pseudocurvisetus, Rhizosolehia imbricata, Thalassiosira subtilis, Rhizosolenia sinensis and T. nitzschioide;in the investigated sites, the average cell density was 53.80×10⁴ cells/m³, H' was 1.80-3.35, J' was 0.33-0.63, and D was 1.66-3.39. Important environment factors that influencing waters in Haiwei Bay were water tempreture, SiO₄ and NH₄-N; according to RDA, C. curvisetus community structure was positively correlated with SiO₄ and chemical oxygen demand (COD) and C. pseudocurvisetus community structure was positively correlated with water tempreture, COD and pH.【Conclusion】From the summer of 2019 to the summer of 2020, Hainan Haiwei Bay enjoys a good aquatic environment and abundant phytoplankton species. The important environmental factors affecting phytoplankton in Haiwei Bay are water temperature, SiO₄ and NH₄-N. Therefore, after modeling the relationship between environmental factors and phytoplankton population structure in the waters of Haiwei Bay, the changes of SiO4 and NH4-N in the waters of Haiwei Bay should be regularly monitored to avoid red tides. At the same time, the wastewater of mariculture industry and domestic sewage were detected and treated in a targeted way.