Abstract: To investigate the structural characteristics and influencing factors of phytoplankton communities in Dongting Lake, quarterly surveys were conducted in different water areas of the lake in 2023. The results indicated that a total of 44 genera belonging to 6 phytoplankton phyla were identified, including 16 genera of green algae, 12 of diatoms, and 10 of cyanobacteria. The average density of phytoplankton was 8.499×10⁵ cells/L, predominantly composed of diatoms and cyanobacteria, which accounted for 57.1% and 18.6%, respectively. Temporal distribution showed the highest average density in June (1.352×10⁶ cells/L) and the lowest in December (3.448×10⁵ cells/L). Spatially, the highest phytoplankton density was observed in West Dongting Lake (1.419×10⁶ cells/L), mainly comprising diatoms, followed by the river inlet, South Dongting Lake, East Dongting Lake, and the river outlet. The Shannon-Wiener diversity index ranged from 2.76 to 3.62, with the inlet, East Dongting Lake, and outlet rated as excellent, while West and South Dongting Lake were classified as good. The comprehensive nutrient status index for the entire lake was 48.1, indicating a mesotrophic level. Principal Component Analysis revealed that water temperature and chemical oxygen demand were the primary environmental factors. Redundancy Analysis indicated that the main factors affecting phytoplankton distribution were water temperature, dissolved oxygen, and ammonium nitrogen. This study provides data support for the ecological restoration and management of Dongting Lake.