To evaluate the water quality in the middle and lower reaches of the Guo River, the spatial and temporal distribution characteristics of phytoplankton were quantitatively analyzed using species dominance and diversity indices methods based on four measuring data samples of phytoplankton and water quality from December 2016 to September 2017. The results show that the phytoplankton distributed unevenly in the main stream and differed greatly among different river sections. A total of 7 species and 89 genera of phytoplankton were found in the Guo River, in which Chlorophyta and Bacillariophyta were the main species. Furthermore, the dominant species of phytoplankton varied greatly with the seasons. A total of 16 dominant species were found in the four samples and those of Chlorophyta were the most in all four seasons. Shannon-Wiener diversity index evaluation grade reflects that the overall trend of water pollution was gradually improved. In addition, redundancy analysis results display that the environmental factors explained 60. 02% of the overall variation in spatial and temporal distributions of the phytoplankton. Moreover, temperature, conductivity and total phosphorus were the main environmental factors affecting spatial and temporal distributions of phytoplankton in the middle and lower reaches of the Guo River.