Abstract: A one-year survey studying on species composition and cell density of phytoplankton in Hua’an reaches of Jiulongjiang River was carried out in 2013.The result showed that there were 57 species of phytoplankton in the investigated waters, which belonged to 6 phyla of phytoplankton including Bacillariophyta, Chlorophyta, Euglenophyta, Cryptophyta, Pyrrophyta and Cyanophyta. The species of Bacillariophyta had the relative advantage, accounting for the total average cell density of 69.61%. Chlorophyta took the second place, accounting for 12.89%.Following by were Euglenophyta and Cryptophyta, accounting for 7.22% and 6.51% respectively. Pyrrophyta and Cyanophyta were the least, accounting for 1.98% and 1.80% respectively. The main dominant species included Melosira granulata var. angustissima, Nitzschia sp., Synedra acus, Euglena sp.,Gyrosigma sp.,Cryptomonas spp.and Peridinium sp.,ect. The average cell density of phytoplankton at all investigation stations was 28232 cells/L,with a range of 3 700 ～12 500 cells/L,29 113 cells/L in Xipi reservoir,31 863 cells/L in Mianliang reservoir, 23 721 cells/L in Chengguan reservoir ,respectively. Average diversity index and evenness were 2.91 and 0.76 respectively.The annual change of the two indices was similar at each same station, and there was a certain correlation between the two indices. The annual change of cell density of phytoplankton was related to water temperature and flood season obviously, it was the lowest in January, May and July, and the highest in March. Peridinium sp. became the dominant species in some stations in March, and had the possibility of dinoflagellate bloom. According to the variation of cell density of phytoplankton and the need of controlling bloom in spring and winter, we thought autumn was suitable for the release and enhancement.