Abstract: Based on phytoplankton survey date in September 2013 in Maoming,Zhuhai and Lufeng city,the species composition,abundance distribution,dominant species and diversity of phytoplankton were analyzed.Results showed that a total of 112 phytoplankton species were identified in four phylum.87 species of Bacillariophyta,accounting for 77.68%,17 species of Pyrrophyta,accounting for 15.18%,5 species of Chlorophyta,accounting for 4.46%,3 species of Cyanophyta,accounting for 2.68% respectively.Phytoplankton in Maoming was 52 species,in Zhuhai was 67 species,in Lufeng was 63 species.The abundance of phytoplankton was highest in Maoming sea,with an average of 356.21×10⁴cells·L^-1,followed by Zhuhai sea,with an average of 25.13×10⁴cells·L^-1,without doubt Lufeng sea was lowest,with the average of 1.66×10⁴cells·L^-1.Ten dominant species appeared in three sea area,and the composition of dominant species were different in each area,only Peseudo-nitzschia delicatissima appeared in all survey sea.Dominant index was very high in Maoming,as high as 0.99,followed by Lufeng 0.59.Shannon-Wiener diversity index H',Pielou evenness index J' and Margalef richness index D were higher in Zhuhai (2.98,0.60,1.13) and Lufeng (1.57,0.35,1.03) than Maoming (0.11,0.03,0.73).Biodiversity index comprehensive evaluation displayed that the water quality was good in Zhuhai sea and the phytoplankton community structure was stable,while in Maoming sea was bad.