Qieyeshi reservoir, a drinking water supply reservoir, was investigated to evaluate its trophic state based on TSI index in flood and dry seasons of 2000. The physical and chemical factors such as temperature and nutrients are measured. The composition and abundance of plankton comunnity are analyzed, the results show that the reservoir was eutrophic, the TSI index was higher in the dry season that in the flood season. The pollution resulting from organic matter was very serious. Cyanophyta and Chlorophyta predominated numerically in the flood season and then Cyanophyta and Bacillariophyta in the dry season, organic-resistant Euglenophyta was well represented. The abundance of zooplankton was predominated by rotifer and copepoda.The predominated species of rotifer were organic matter-resistant and eutrophic-resistant species such as Brachionus Icalyciflorus, Asplauchna priodonta and Keratella cochlearis. The nauplii and cope-podite of copepoda abundantly presented in the reservoir but the adults was scarce. The numbers of species and abundance of Cladocera was low and predominated by thermophilic and eutrophic-resistant species such as Bosmina longirostris and Moina brachiata.