Abstract: Based on two surveys respectively in August 2013 and May 2014 in Caofeidian coastal waters, phytoplankton community, its correlations with environmental factors, and changes after land reclamation were analyzed.The results showed that totally 49 and 76 species were identified in spring and summer, respectively, in which diatoms dominated with rare dinoflagellates and chrysophytes.Phytoplankton abundance in spring ranged from 1.93×10⁴ cells/m³ to 7.89×10⁴ cells/m³, and dominant species included Paralia sulcata, Coscinodiscus subtilis, etc.; while in summer, it ranged from 5.85×10⁴ cells/m³ to 5022.91×10⁴ cells/m³, and dominant species were Eucampia zodiacus, Skeletonema costatum, etc.The distributions of phytoplankton abundance in spring and summer were similar with high abundances ordinarily observed in inshore waters.Temperature, salinity and nutrients were important factors controlling seasonal shifts and spatial variations of phytoplankton community.After land reclamation, the changes of Shannon-Wiener diversity index (H') and Pielou eveness index (J) showed spatial variations, which was related to nutrients redistribution due to hydrodynamic changes.The northeastern bay was seriously affected by reclamation.Freshwater and nutrients imported by rivers were hard to exchange outside, thus promoting the proliferation of E.zodiacus whose abundance reached 3725.58×10⁴ cells/m³ in summer, which suggested that the bay tended to be threatened by red tide.