Xiangxi River originates from the Shennongjia Forest region and is the largest tributary in Hubei Province of Three Gorges Reservoir. Three reservoirs(Gudongkou Reservoir 1, Gudongkou Reservoir 2 and Xiangxi Bay of Three Gorges Reservoir) in Xiangxi River Basin were selected to investigate phytoplankton ecology in such a reservoir cascade system. In this study, phytoplankton composition, dominant species, community structure, abundance and biodiversity index were investigated and analyzed from October 2005 to July 2006. A total of 121 species, belonging to 58 genera in 7 phyla were identified, among which Chlorophyta(49 species of 26 genera) is the most important compontent, accounting for 40.50% of the total; followed by Bacillariophyta (41 species of 14 genera) for 33.88%, and Pyrrophyta(11 species of 3 genera) for 9.09%, Cyanophyta(7 species of 5 genera) for 5.79%, Cryptophyta(7 species of 3 genera) for 5.79% and other species for 4.96%. 31 species of 25 genera, 40 species of 29 genera and 81 species of 46 genera were found in Gudongkou Reservoir 1, 2, and Xiangxi Bay, respectively. The dominant species in Gudongkou Reservoir 1 belonged to Bacillariophyta, Chlorophyta, while Bacillariophyta, Cryptophyta and Pyrrophyta dominated in Gudongkou Reservoir 2. Chlorophyta, Bacillariophyta, Pyrrophyta, Cryptophyta also played an important role in Xiangxi Bay. The annual abundance of phytoplankton community in the three reservoirs was 1.110×10⁶, 4.837×10⁵ and 1.734×10⁶ cells/L, respectively. It had highest density in Xiangxi Bay(4.87×10⁶ cells/L) and lowest density in Gudongkou Reservoir 2(5.76×10⁵ cells/L). Principal Component Analysis was applied to analyze the water quality of the three reservoirs. The result implies that the water quality was degrading along the cascade of reservoirs. There were no significant differences in diversity index(H) and evenness(J) among the three reservoirs, while the Margalef index of Xiangxi Bay was significantly larger than that of Gudongkou Reservoir 1 and Gudongkou Reservoir 2. The valuation results by the first two indices and dominant phytoplankton species showed that the cascade reservoirs in Xiangxi River Basin were of intermediate pollution state.