Abstract: The community structure and succession of phytoplankton are strongly related to water quality and affects the structure, function and utilization of aquatic ecosystem, especially in reservoirs. Seasonal annual surveys on 10 typical reservoirs in hilly regions of Sichuan Province were conducted from 2016 to 2017, with the aim to explore the annual changes, succession rules and influencing factors of phytoplankton community structure in this area. Phytoplankton functional classification and redundancy analysis were used. 24 phytoplankton functional groups were identified in total, which are dominated by the 6 groups of S1, T_C, S_N, S2, L_O and J. Furthermore, the seasonal succession patterns for the dominant functional groups of phytoplankton in investigated reservoirs were identified as follows: S1+T_C+S_N+L_O+S2 in summer, S1+S2+J+S_N+X1+L_O+X2 in autumn, S1+X2+C+X1+J+Y in winter and S1+S_N+H1+J+X1+X2 in spring. The results showed that the phytoplankton is always dominated by filamentous cyanobacteria whose composition changed from season to season. The formation and breaking of reservoir stratification, changes of light intensity, grazing intensity and nutrient level in water in different seasons led to the fluctuation of phytoplankton functional groups. Redundancy analysis indicated that water temperature, total nitrogen, total phosphorus, ammonia nitrogen, Nitrate nitrogen, permanganate index and Secchi depth were the main environmental factors that influencing the functional groups. Although the filamentous cyanobacteria were seasonally different in terms of its dominant species, its abundance increased with the increase of nutrient level and water temperature. In order to ensure the safety of life and production in this area, it is recommended that the water quality of the reservoirs should be effectively managed and immediately repaired to prevent the explosive proliferation of filamentous cyanobacteria under the dual promotion of high nutritional level and global warming.