Sichuan is known as a province with thousands of rivers and numerous reservoirs which play an important role in production and life. There is a general problem of pollution in small and medium-sized reservoirs in this area, which may affect the structure, function and utilization of aquatic ecosystem. Aiming to explore the annual changes and succession rules of phytoplankton community structure in small and medium-sized reservoirs in hilly regions of Sichuan Province and to predict its development direction, this study conducts an annual survey on 10 typical reservoirs in specific regions in different seasons from 2016 to 2017. The survey identifies a total of 188 species of phytoplankton in 104 genera of 9 phyla, including 16 species (9 species are cyanobacteria) of 16 genera of 4 phyla which are identified as dominant species. In particular, the dominance index of Pseudoanabaena linmnetica is the highest, and the density of cyanobacteria is dominant in all seasons and reservoirs, especially in summer. Furthermore, the improved Levins formula and Petraitis index are applied to determine the niche metrics of the phytoplankton dominant species. The results show that the niche breadth and niche overlap of each dominant species vary seasonally and are significantly affected by water temperature and nutrient. Based on their niche breadth in different seasons, the dominant species are classified into three categories, two of which are relatively competitive. These two categories include 7 species, of which there are 5 species of cyanobacteria, including P. linmnetica. The dominant species in four seasons are mostly in a state of developing, and the most promising species throughout the year are mainly cyanobacteria. In particular, species with the potential to produce toxins such as P. linmnetica have the risk of further expansion of their dominance. The results of interspecific association show a positive correlation among the community species throughout the year, indicating that the community structure in these reservoirs is relatively stable and there is a positive succession trend. In such a trend, intermittent toxic cyanobacteria bloom which affects the quality and function of water may occur in these reservoirs in summer, especially in reservoirs with simple dominant species composition.This paper provides research background for the study of phytoplankton community succession and the early warning of cyanobacteria blooms in small and medium-sized reservoirs in hilly regions of Sichuan Province. To prevent possible filamentous cyanobacteria bloom outbreak, water quality of the reservoirs should be effectively managed and immediately repaired under the dual promotion of high nutritional level and global warming.