Based on the results of phytoplankton and water quality monitoring in a new reservoir in Shandong Province from March 2014 to December 2017, the composition, density and diversity of phytoplankton were clarified. Non-metric multidimensional scaling was used to analyze the difference of phytoplankton community structure in different years and seasons. The relationship between phytoplankton community structure and environmental impact factors was explored by redundancy analysis. The results showed that Synedra acus, Pseudanabaena limnetica and Chlorella vulgaris were dominant algae species in this reservoir. The seasonal succession of the phytoplankton community structure in the reservoir was not stable. In winter and spring, Bacillariophyta and Cryptophyta were dominant, but phytoplankton community in summer and autumn were quite different in different years and Bacillariophyta competed with Cyanophyta in these two seasons. The reservoir started to activate at the end of 2013,due to fluctuations in the water environment, the community structure changed by the time. But the phytoplankton community structure of the reservoir was generally changed from Cyanophyta-Cryptophyta-Bacillariophyta to Bacillariophyta-Cryptophyta-Cyanophyta. The growth of phytoplankton in this reservoir was mainly affected by nitrogen and COD_Mn, however, main environmental factors were different in different years.