Abstract: In order to understand the current situation of phytoplankton in the lower reaches of the Yalong River after the operation of cascade hydropower stations, 22 sections of the lower reaches of the Yalong River were sampled and investigated for phytoplankton and environmental factors in the spring and autumn of 2020, to analyze the spatial-temporal variation of phytoplankton community structure and explore environmental driving factors. A total of 123 species (including varieties) belonged to 7 phylum, 10 classes, 20 orders, 36 families and 72 genera were identified. They were mainly diatom, chlorophyta and cyanophyta. The number of species in spring was significantly higher than that in autumn, and there were 55 common species. The number of species in main stream was similar to that in tributary, and there were 82 common species. There were five dominant species in the whole year, namely Cyclotella meneghiniana, Synedra acus, Achnanthidium minutissima, Chroomonas acuta and Raphidiopsis sp. There were five dominant species in spring, four in autumn, six in main stream and four in tributary. During the investigation, the average abundance of phytoplankton in the lower reaches of Yalong River was 3.17×106  cells/L. The average abundance in autumn was higher than that in spring. The average abundance in four tributaries was higher than that in main stream. Correlation analysis showed that TP, SS, CODMn, DO, SD, WT, NO3-N, Chl.a, TN and EC were significantly correlated with different dominant phytoplankton species. RDA results showed that the environmental driving factors in spring were TN, PO4, WT, SD and DO. The main stream sections were divided into two parts, and the tributary sections were obviously separated. The environmental driving factors in autumn were WT, SD and SS.