The annual dynamics of phytoplankton community structure in Lake Caizi has been investigated in 2010. The results indicated that:(1) A total of 285 phytoplankton species from 110 genera of 8 phylum were identified. Phytoplankton species composition in different months showed significant difference. The maximal number of phytoplankton species (173) occurred in March while the minimum (105) in January. The dominant class of phytoplankton changed seasonally. Cyanophyta peaked all the year round; diatoms had an apparent dominance in January, May, September and November; Xanthophyta also played an important role in January, March and May; Chlorophyta dominated in November, Cryptophyta dominated in May while Chrysophyta dominated in January. The cell density and biomass of phytoplankton were higher in summer and autumn than that in winter and spring. The cell density of phytoplankton between different months had significant difference. The maximal density, with the value (66.13 ± 8.58)×10⁵ cells/L occurred in July while the minimal value of (12.78 ± 0.61)×10⁵ cells/L appeared in January. Whereas, the maximal biomass of phytoplankton(2.80 ± 0.17 mg/L) occurred in September and the minimal(0.72 ± 0.03 mg/L) appeared in May, and the biomass in different months was also significant different. (2) Temporal variations of three indices (including Margalef index, Shannon-Wiener index and Pielou evenness index) were obvious. All the indices were higher in winter and spring than those in summer and autumn. The maximal value occurred in March while the minimal one appeared in July. (3) The phytoplankton community structure was influenced by different factors in different months. As a result, the groups of sampling station changed seasonally according to cluster analysis. (4) The obvious variation of phytoplankton community structure was present in 2010 compared with that in 2007. The number of species decreased from 340 in 2007 to 285 in 2010, however, the cell density increased obviously from (5.91 ± 0.90)×10⁵ cells/L in 2007 to (33.81 ± 10.10)×10⁵ cells/L in 2010. At the same time, both the numbers of oligotrophic algae and perphytic algae decreased to the same extent and eutrophic, filamentous and planktonic algae increased.