Abstract: To reveal the composition characteristics of the phytoplankton community in the nearshore waters of Changjiang, Hainan, and its relationship with environmental factors, a systematic study on the seasonal dynamic characteristics of the phytoplankton community in this sea area was conducted from August 2023 to March 2024. The aim was to provide a theoretical basis and data support for the protection of the regional ecological environment and the rational development and utilization of biological resources. A total of 149 species of phytoplankton belonging to 4 phyla and 54 genera were identified in this survey. Among them, Bacillariophyta dominated, followed by Dinophyta, while Chrysophyta and Cyanophyta accounted for relatively low proportions. Analysis of dominant species indicated that Skeletonema costatum, Leptocylindrus danicus, and Thalassionema nitzschioides consistently occupied dominant positions in all four seasons. Thalassionema nitzschioides was distributed in all four seasons and was the main dominant species in the phytoplankton community of this sea area. Phytoplankton abundance showed spatial significant heterogeneity and seasonal differences, with an average abundance of 1.02×10⁴ cells/dm³ at each station. The abundance variation trend was winter > autumn > spring > summer. In terms of spatial distribution, both spring and winter showed a decreasing trend from the nearshore to the open sea, with high-value areas concentrated in the sea area from Haihua Island to Yangpu Bay. In summer, the high-value area shifted southward to the Qizi Bay area, while in autumn, the high-value area expanded towards the northwest open sea. The diversity index (H') ranged from 0.92 to 3.88, the richness index (d) was 0.15–2.80, and the evenness index (J') was 0.40–0.99, indicating that the water quality in the Changjiang sea area was good. The results of redundancy analysis (RDA) and Pearson correlation analysis indicated that water temperature, salinity, and chemical oxygen demand (COD) were the main environmental factors driving the changes in the phytoplankton community structure in the Changjiang sea area. Among them, water temperature and salinity mainly regulated the distribution characteristics of dominant diatom species in autumn and winter, while COD showed significant driving effects in all four seasons. Compared with the historical data from 2008 to 2009, the seasonal abundance ranking peaked in autumn and then rose to its highest in winter, indicating that the phytoplankton community in this area has changed. Based on those results, further strengthening monitoring of red tides and eutrophication in this sea area is necessary to promote the sustainable development of the marine ecological environment in this region.