Jinshuitan Reservoir is an important hydroelectric project in Oujiang River which is the second largest man-made river in Zhejiang Province. Its eco-environment and water management have a great significance on sustainable development of this area. With the fast economical development of the area, the water quality has been declined as an amount of pollutants discharged. Most of previous research works on Jinshuitan Reservoir watershed mainly focused on its meteorology, climatology and aquaculture, lacking of phytoplankton community structure, seasonal variation and water quality assessment. To explore the characteristics of the phytoplankton community structure and the response of water quality, which can provide database for its water utilization and protection, a survey was carried out every alternative month from January to November of 2010. Based on the analysis of phytoplankton samples from six sites in Jinshuitan Reservoir , the characteristics of community structure and abundance distribution were studied. Cell enumeration and species identification were performed by light microscope (Olympus BX-51). The dominant population, species diversity and community structure of phytoplankton were determined as well. A total of 284 species in 105 genera and 7 divisions were identified, of which the most abundant group was Chlorophyta, with 139 species in 51 genera that account for 48.94% of total species, followed by Bacillariophyta (67 species in 19 genera), Cyanophyta (52 species in 22 genera), Chrysophyta (9 species in 4 genera), Dinophyta (8 species in 5 genera), Euglenophyta (5 species in 2 genera), and Crytophyta (4 species in 2 genera). The abundance of phytoplankton cells varied from 1.04×10⁵ to 3.70×10⁶ cells/L (averaging 9.63×10⁵ cells/L). The indices of Shannon-Wiener, Margalef's and Pielou Evenness were 1.76-4.64 (averaging 3.09), 0.48-2.80 (averaging 1.62) and 0.51-1.26 (averaging 0.91), respectively. The dominant species of each season were identified and their contribution to the total abundance was analyzed. The seasonal variation of phytoplankton community structure was significant. The relationship between phytoplankton and environmental factors were discussed. By using Pearson correlation analysis, nutrients and temperature were the essential factors to determine the seasonal changes of phytoplankton species biodiversity and cell abundance. The nutrients were the key factor to affect the horizontal distribution and cell abundance of phytoplankton. Cell abundance is postively correlated with total phosphorus(TP) and Chlorophyll-a(r=0.292 and 0.143, respectively, P< 0.01). It is negatively correlated with temperature, total nitrogen(TN) and transparency(r=-0.605, -0.384 and -0.18 respectively, P< 0.01). The number of species exhibited a negative correlation with temperature, TN and TP(r=-0.187, -0.3 and -0.466 respectively, P< 0.01). TSI(Σ), dominant species, Shannon-Weaver diversity index, Margalef index and Pielou evenness index were integrated to assess the water quality. The result indicated that the water quality of Jinshuitan Reservoir was medium-eutrophicated.