Abstract: The present study investigated the seasonal variation characteristics of phytoplankton community structure and its correlation with environmental factors in Qinzhou bay based on four cruises carried out from 2013 to 2014. The results showed that total of 148 species, including forma and varieties, which belonged to 8 phyla and 71 genera were identified. Amongst, species in Bacillariophyta are in the majority, and species in Pyrrophyta are in the minority. In our cruises, most dominant species belonged to eurythermal diatoms. It was noteworthy that Phaeocystis globosa was considered a dominant species due to the algal bloom in the cruise of December 2014. Except for December 2014, phytoplankton community during the other three cruises showed high community diversity index and evenness index thereby suggested that the community structure of phytoplankton was comparatively stable in Qinzhou bay. The Phaeocystis globosa bloom did damage on the structure of phytoplankton community and stability of ecosystem. It was closely related to environmental conditions. The multivariate statistical analysis of phytoplankton community in Qinzhou bay was carried on by using PRIMER 6 software. The result showed that there were significant differences in phytoplankton community structure during four cruises. There was correlation between phytoplankton abundance and environmental factors. There was a good correlation between abundance and nitrogen in 2013-11 and 2014-12. The correlation between abundance and inorganic phosphorus was significant in April, 2014.The correlation of the temperature and salinity was shown in 2014-08. In recent years, the structure of phytoplankton community in Qinzhou bay is relative stable.