Sampling was carried out in March and July 2008 in order to study the strucaure of phytoplankton community and its re-lationship with environment factors in different seasons in Lake Honghu. Total 6 phylum, 46 genera and 95 species of phytoplank-ton were identifed. The phytoplankton abundance ranged from 2.00x10⁵to284x 10⁵cells/I.Bacillariophyta was the most abun-dance phylum, following as Chlorophyta and Cyanophyta. Fragilaria, Scenedesmus and Melosira were the dominant genera. of Al-gae community structure was different between dry seasons and wet seasons. Due to adaptability of low water temperature and lightintensity in dry seasons, diatoms were predominant phylum. Diatoms were also predominant phylum in wet seasons due to abundantSi in water. Principal Components Analysis(PCA) indicated that the kinds of ions and pollutants in the water body were the mostimportant environment factors in the lake. Canonical Correspondence Analysis(CCA) illuminated water temperature, dissolve oxy-gen and suspended solids could he the main forces structuring the phytoplankton community in the Lake Honghu.