Abstract: Background The rapid development of aquaculture in the waters of Changhai County has promoted regional economic growth while imposing certain environmental pressure on the coastal marine ecosystem. As sensitive indicators of aquatic environmental changes, phytoplankton are critical for evaluating marine ecological quality. Investigations on phytoplankton communities can objectively reflect the current ecological status of regional waters. Objective This study aimed to clarify the community structure, seasonal succession, and diversity characteristics of phytoplankton in the adjacent waters of Changhai County, and identify the major environmental factors driving phytoplankton community variation, so as to provide basic references for regional aquatic ecological monitoring and conservation. Methods Monthly synchronous surveys of phytoplankton and water environmental parameters were conducted from March to November 2023 in the waters of Changhai County. Sample collection and parameter determination were performed in accordance with national marine survey specifications, and ten environmental factors were measured, including water temperature, salinity, dissolved oxygen, and nutrients. Phytoplankton community characteristics were analyzed using Shannon-Wiener diversity index (H), Pielou evenness index (J), Margalef richness index (D), and Eutrophication index (E). Pearson correlation analysis and redundancy analysis (RDA) were applied to explore the relationships between phytoplankton communities and environmental factors. Results The phytoplankton community was dominated by Bacillariophyta and Dinophyta, among which Bacillariophyta showed absolute predominance with distinct seasonal succession patterns. Phytoplankton abundance exhibited significant seasonal variation, while the overall community diversity remained relatively low. The surveyed waters were in good condition without eutrophication. Salinity, water temperature, dissolved oxygen, and chemical oxygen demand were the core factors regulating phytoplankton community structure. Phytoplankton abundance and diversity were significantly correlated with key environmental factors such as water temperature and salinity. The water environment in spring was suitable for diatom proliferation, accompanied by weak community stability and potential risks of algal bloom outbreaks. Conclusion The phytoplankton community in the waters of Changhai County presents obvious seasonal succession and is co-regulated by multiple water environmental factors, with potential seasonal ecological risks. This study supplements basic ecological data on local phytoplankton and provides a scientific reference for marine ecological management, red tide prevention, and coordinated ecological and aquacultural development in Changhai coastal waters.