According to the survey data of 8 voyages in Haizhou Bay and its adjacent waters in spring and summer of 2015—2018, community structure of phytoplankton was studied and their relationships with environmental factors were evaluated by pearson correlation analysis and CCA. A total of 96 species of phytoplankton were found, including 80 species of diatoms. Skeletonema costatum was the first dominant species in all voyages. Cell abundance ranged from 4.20×10³cells/L to 7 952.00×10³cells/L with an average of 504.54×10³cells/L. Average H’ was 2.66. The lowest H’ was 1.52 in the summer of 2018, and the highest H’ was 3.73 in the spring of 2015. Diversity basically showed a trend of high in spring and low in summer. Pearson’s correlation analysis indicated that phytoplankton abundance were positively correlated with water temperature，active phosphate and chemical oxygen demand. CCA suggested that the dominant species in Haizhou Bay were mainly affected by water temperature, dissolved oxygen and salinity in spring, while in summer they were affected by many factors. The phytoplankton diversity in Haizhou Bay has declined slowly in recent years, which may be affected by largescale laver culture.