Abstract: Abstract：In order to study the structure and spatial and temporal distribution of phytoplankton communities in the main stream of Jinsha River, 14 sections of the main stream were investigated and analyzed in the spring and autumn of 2017-2018. The results showed that there were 100 species (including varieties) of phytoplankton in 8 phyla in the main stream of Jinsha River, mainly composed of diatoms (56%), cyanobacteria (22%) and green algae (11%). The density of phytoplankton ranged from 0.019 3×106 to 0.341×106 cells/L, and the biomass of phytoplankton ranged from 0.016 to 0.434 mg/L. There were significant differences among the river sections. Shannon diversity index ranged from 1.168 to 2.519, Pielou evenness index ranged from 0.559 to 0.876, Margalef index ranged from 1.573 to 3.36, and The diversity index of each river section varies. The water environment of Jinsha River mainstream was moderately polluted by comprehensive evaluation. The results of hierarchical clustering and ordination showed that the sample plot communities in spring and autumn of 2017 and 2018 were divided into three groups. The upper, middle and lower reaches show similarities in different seasons and years, and the spatial and temporal differences of phytoplankton communities are caused by the environmental differences of different sections. In time, the phytoplankton community structure in the main stream of Jinsha River has obvious succession in each season, both continuation and alternation; in space, the more uniform and vigorous the phytoplankton community is from upstream to downstream, the more obvious the spatial heterogeneity is. The cluster and ordination of phytoplankton revealed the distribution of dominant species, the number of species and the similarity of habitats.