From March to October in 2010, through sampling in spring, summer and autumn, the composition of phytoplankton species and its changes of Taihu Lake were investigated, which found that there were92 genera and 279 species of phytoplankton in Taihu Lake. Common species of the phytoplankton could change with seasonal changes, however the spatial variation in composition of phytoplankton species was small. We preliminarily explored the spatial variation rule of composition of phytoplankton species in Taihu Lake. This result provided technical support for early warning monitoring and the protection of water environment.