Abstract: Based on the survey data collected from Shuidong Bay in April and October, 2013, we analyzed the species composition, spatial and temporal distribution and diversity of phytoplankton. A total of 69 species of phytoplankton belonging to four phylums were identified (59 species of Bacillariophyta, accounting for 85.51%; 8 species of Pyrrophyta, accounting for 11.59%; 1 species of Cyanobacteria and 1 species of Chrysophyta, accounting for 1.45%). The abundance of phytoplankton in autumn (203.10104 indm-3) was 8.87 times higher than that in spring (22.89104 indm-3), and the abundances obtained from the sampling stations outside the bay and inside the bay in spring and autumn were high. Skeletonema costatum was the first dominant species in spring and autumn, and the dominances were 0.64 and 0.54, respectively. Shannon-Wiener diversity index (H) and Pielou evenness index (J') were higher in spring (2.38, 0.66) than in autumn (1.72, 0.43). Margalef richness index (D) was lower than 1.00 in the two seasons. It is concluded that the Shuidong Bay had been polluted.