Abstract: According to the monitoring of phytoplankton samples including red tide organisms in Pingtan inshore sea area from April(spring) to June(summer) in 2021,the ecological characteristics of phytoplankton and the Pearson correlation with environmental factors were analysed,in order to explore the changes of red tide organisms in Pingtan inshore sea area. The results showed that a total of 130 phytoplankton species in 4 phyla were identified,which was dominated by diatoms. Species quantity of phytoplankton was in the order of June>May>April. The 13 main dominant species were Skeletonema costatum,Pseudo-nitzschia delicatissima,Chaetoceros curvisetus,Leptocylindrus danicus,Melosira sulcata,Prorocentrum donghaiense,etc. The peak value of phytoplankton cell density was in June. The density ranges from 2.17×10⁴ to 2.11×10⁵ cells/L at different stations,with an average of(6.04~9.85)×10⁴ cells/L. The overall evaluation was in the middle level with the highest level at CCPT04 station in Liushui sea area and the lowest at CCPT02 in Suao sea area. According to the Pearson correlation analysis,water temperature,pH and salinity were main factors affecting the phytoplankton community. Red tide disaster prevention should be strengthened in the period of high occurrence of red tide.