Abstract: We analyzed the species composition, dominant species, abundance and community diversity of phytoplankton based on the data collected from the ecological investigation of the Nansha sea area of Pearl River Estuary in both wet season (April, 2015) and dry season (November, 2015).A total of 76 species among 44 genera belonging to 5 phyla were identified in the survey area. Diatom was the predominant group of all the phyla. In wet season there were 2 dominant species,  Cylindrotheca closterium and Skeletonema costatum. In dry season, there were 3 dominant species, Thalassiosira mala, S.costatum and Coscinodiscus spinosus. The average cell abundance of phytoplankton in wet and dry seasons was 651.91×10⁴cells · L^-1 and 129.21×10⁴cells · L^-1, respectively. The cell abundance distribution during the two seasons showed an increasing trend from inshore to offshore. In general, the Shannon-Wiener index (H′) and the Pielou evenness index (J′) were 1.64 and 0.40 in wet season, and 1.58 and 0.41 in dry season, suggesting low diversity of the phytoplankton community. Ordination diagram of CCA results indicate that salinity, temperature, DIN and pH are the main environmental factors which affect phytoplankton community in wet season, while temperature, transparency, phosphate and salinity are the main affecting factors in dry season.