Abstract: In order to explore the temporal and spatial evolution characteristics of phytoplankton in a typical curved braided river, three in-situ observations were carried out in Guanzhou reach of the middle reaches of the Yangtze River from September 2020 (wet season), December 2020 (dry season) to May 2021 (normal season). Six observation sections of three types of straight, slightly curved and branched were set up to investigate and analyze environmental factors, nutrients and phytoplankton. The results showed that a total of 86 species of phytoplankton belonging to 47 genera and 6 phyla were collected in the study area, of which Bacillariophyta and Cyanophyta were the dominant species. In terms of temporal changes, the species of phytoplankton increased gradually from the wet season to the normal season of the next year, but the density of algae decreased significantly. The species and density of phytoplankton were significantly affected by the hydrological periods. In terms of spatial distribution, the curved braided river had the largest number of phytoplankton species and its abundance accounted for 34.04 % of the whole Guanzhou area. The relative abundance of phytoplankton in the head, middle and tail of Guanzhou (transitional river and curved braided river) was higher than that in the straight section. The Guanzhou section was affected by the upstream water, and the hydrodynamic forces of the left and right branches changed significantly with the incoming flow conditions, which in turn caused differences in environmental factors and nutrient structure, and drove the local spatial distribution of phytoplankton communities. The pH, water temperature，conductivity, nitrogen and phosphorus in the water were the main driving factors of the phytoplankton in the curved bifurcation section of Guanzhou. The influence of water environmental factors on phytoplankton was more significant.