Abstract: To reveal the structural characteristics of the phytoplankton community in Tuohu Lake and its relationship with environmental factors, we analyzed the phytoplankton species composition, abundance, biomass, and environmental factors at 11 sampling sites in July (summer) of 2016. A total of 96 phytoplankton taxa were identified, belonging to 48 genera and eight phyla. The phytoplankton species were dominated by Chlorophyta, with 39 species (40.63% of the total phytoplankton species) from 23 genera, and Bacillariophyta, with 20 species (20.83%) from seven genera. Other notable phyla included:Euglenophyta, with 17 species (17.71%) from five genera; Cyanophyta, with 14 species (14.58%) from eight genera; Pyrrophyta, with two species (2.08%) from two genera; Cryptophyta, with two species (2.08%) from one genus; and Xanthophyta and Chrysophyta, both with one species from one genus, accounting for 1.04% of total phytoplankton species, respectively. Given their dominance in the community structure, the phytoplankton species composition was Chlorophyta and Bacillariophyta type. In addition, the sampling sites at the lower reaches exhibited higher phytoplankton cell abundance and biomass than that observed in the upper reaches. The average cell density and biomass of phytoplankton were 4.022×10⁶ cells/L and 3.046 mg/L, respectively. Cyanophyta and Chlorophyta cell abundance was highest, whereas Bacillariophyta and Euglenophyta biomass was highest. The Shannon-Wiener and Pielou indices of phytoplankton in the upstream sampling sites were higher than those downstream. Furthermore, the water quality of Tuohu Lake was determined to be in oligotrophic-β mesotrophic status. Canonical correspondence analysis showed that conductivity, transparency, water depth, and pH were strongly correlated with the phytoplankton community structure.