Phytoplankton and environmental factors were investigated at 8 sampling sites in the Lake Baiyangdian in May and August of 2018, to study their relationship which can provide theoretical basis for water resources management and aquatic ecological protection. The results of Pearson correlation analysis showed that the main environmental factors were dissolved oxygen (DO), permanganate index (COD_Mn), total nitrogen (TN) and total phosphorus (TP) in flood season while those were DO, COD_Mn, ammonia nitrogen (NH₃-N) and TP in non-flood season. Phytoplankton species were detected in 5 phyla, 38 species in flood season and 6 phyla, 43 species in non-flood season, respectively. The density of phytoplankton was 415.30×10⁵-1018.14×10⁵ cells/L and 249.62×10⁵-454.21×10⁵ cells/L, respectively. The dominant species were 6 species and 10 species and were mainly cyanobacteria and green algae. The Shannon-Wiener diversity index (H'), Margalef species richness index (dM), Pielou evenness index (J) and species diversity threshold of phytoplankton community all indicated that the diversity of phytoplankton in flood season was lower than that in non-flood season. H' and J were higher in areas with better water quality (such as the center of the Lake Baiyangdian), whereas H' and J were lower in areas with poor water quality (such as the entrance of Fuhe River and Xiaoyi River). TP and DO were the key factors affecting the characteristics of the phytoplankton community in flood season, while COD_Mn and TP were the key factors affecting the characteristics of the phytoplankton community in non-flood season. The results of water quality evaluation showed that the water quality of the Lake Baiyangdian was in eutrophication and the degree continued to deepen compared with the results of three ecological surveys of phytoplankton since 2005.