Phytoplankton abundance and community structure were investigated and analyzed from January, April to December 2011 in Qingcaosha Reservoir, an important water source in Shanghai, China. Total 8 phyla, 99 genera, 223 taxa of phytoplankton were identified. The phytoplankton abundance ranged from 86.37×10⁴-2594.63×10⁴ cell/L. Cyclotella sp., Anabaena circinalis, Chroococcus minutus, Dactylococcopsis rhaphidioides, Chlamydomonas globosa and Cryptomonas erosa dominated throughout the year. Some species in salt and brackish water appeared occasionally. Cyanobacteria abundance dominated throughout the year, and dominant genera succeed from Chroococcus spp. and Dactylococcopsis spp. in spring, Microcystis spp. in summer, to Anabaena circinalis and Lyngbya spp. in autumn. Canonical Correspondence Analysis (CCA) indicated that water temperature, suspended solid and grazing effect of zooplankton could be the main factors shaping the phytoplankton community in the Qingcaosha Reservoir.