[Objective] Phytoplankton community structure and its responses to environmental factors are key indicators for assessing riverine ecological health and formulating effective water management strategies. This study aims to elucidate the spatiotemporal dynamics of phytoplankton communities and their environmental drivers in the Ji’an section of the Ganjiang River. [Methods] A total of 13 sampling sites were surveyed in July (wet season) and November (dry season) in 2023. Phytoplankton samples were collected using plankton nets and water samplers, and species identification and enumeration were conducted through microscopic analysis following standard ecological protocols. Environmental parameters, including water temperature (WT), pH, dissolved oxygen (DO), total nitrogen(TN), total phosphorus(TP), ammonium nitrogen(NH₄⁺-N), and permanganate index (COD_Mn), were measured according to national standard methodologies. Multivariate analyses, including principal coordinates analysis(PCoA), Spearman correlation analysis, and redundancy analysis(RDA), were performed to characterize the phytoplankton community composition and identify key environmental drivers. [Results] 1) The investigation identified a total of 222 phytoplankton genus/ species belonging to 8 phyla. Bacillariophyta and Chlorophyta were the most abundant groups, collectively constituting a typical diatom-green algae assemblage indicative of moderate ecological conditions. Cyclotella sp. was the consistently dominant species across both seasons, underscoring its wide ecological adaptability in the river ecosystem. 2) Seasonal variations markedly influenced community structure; phytoplankton cell densities were significantly higher in July compared to November, whereas diversity indices, including Margalef(d), Shannon-Wiener(H'), and Pielou(J), were substantially lower in July, reflecting decreased community stability during the wet season. 3) Spatial differences in community composition among sampling sites were not statistically significant, indicating the dominance of seasonal rather than spatial effects. Nevertheless, a significant distance-decay relationship was detected, which was notably stronger during the dry season (November), likely due to reduced hydrological connectivity limiting phytoplankton dispersal. SIMPER analysis further confirmed significant compositional differences between seasons, highlighting genus/ species such as Chroomonas sp., Chlorella sp., Merismopedia minima, Chlamydomonas sp., Cryptomonas sp., and Scenedesmus quadricauda as primary contributors to seasonal community variation. 4) Correlation analyses revealed that phytoplankton community characteristics were significantly associated with environmental factors, including pH, TN, TP, and COD_Mn. RDA results specifically identified WT, TN, COD_Mn, and DO as the principal environmental factors driving the distribution and abundance of dominant phytoplankton species. Higher water temperatures significantly promoted the proliferation of cyanobacteria and green algae, whereas lower temperatures favored diatoms and cryptophytes. These findings underscore the critical role of seasonal variations and nutrient dynamics in structuring phytoplankton communities within the Ganjiang River. [Conclusions] Consequently, this study recommends enhancing season-specific water quality monitoring and adopting integrated management approaches, such as reducing nutrient loading from agricultural and industrial sources, implementing ecological buffer zones along riparian areas, and applying artificial aeration strategies, to mitigate eutrophication risks and ensure the sustainable ecological health of the river ecosystem.