Abstract: To explore the characteristics of phytoplankton community structure and their relationships with influencing factors in the trunk stream and main tributaries of the Tanjiang River, the phytoplankton, land use types and water environmental factors of 23 sampling sections were investigated in December 2021 and June 2022. A total of 6 phyla and 288 species of phytoplankton were detected, and the number of phytoplankton species in dry season was higher than those in wet season. Scenedesmus bicauda, Scenedesmus quadricauda, Scenedesmus bijuga, Chlorella vulgaris, Cyclotella catenata, Microcystis aeruginosa, Cryptomonas ovata were dominant species during study periods. Average density (14.79×10⁶ cell/L) and biomass (12.18 mg/L) of phytoplankton in wet season were higher than those in dry season (10.92×10⁶ cell/L, 7.69 mg/L). Phytoplankton diversity index showed that water quality in wet season was better than that in dry season. Redundancy analysis (RDA) showed that land use type and environmental factors from 600 m buffer zone had the greatest explanatory ability for phytoplankton community in dry season (61.14%), while land use type and environmental factors from 1200 m buffer zone could explain the changes of phytoplankton in wet season to the greatest extent (59.90%). The results of RDA analysis showed that main influencing factors of dominant species in dry season were nitrogen and phosphorus nutrient salts, water temperature, turbidity, building land and fish ponds area ratio, while main influencing factors of dominant species in wet season were nitrogen and phosphorus nutrient salts, water temperature, turbidity, building land, cultivated land and woodland area ratio. Reasonable allocation of land use types from riparian buffer zone will help to reduce the amounts of pollutant into the river and improve water ecosystem quality.