From June 2008 to May 2009, the phytoplankton community structure characteristics of the net-pen closing area and three corresponding points in Yangcheng Lake were investigated, the relationship between the phytoplankton community structure and environmental variables was analyzed, and multiple parameters were used to evaluate water quality. A total of 275 species, belonging to 116 genera of 8 phyla, were identified. In net-pen closure area, Microcystis incerta, M.aeruginosa and Pseudoanabaena sp. were the dominant species, while Microcystis incerta, Merismopedia aeruginosa,Pseudoanabaena sp. and M.densa were dominant in lake area. The annual mean value of density[(3 143.38±3 660.84)×10⁴ cell/L] and biomass[(3.71±2.31)mg/L] of phytoplankton in the lake area were lower than the average value of density[(3 830.77±4 296.56)×10⁴ cell/L] and biomass[(5.24±4.10)mg/L] in net-pen closure area. The results of redundancy analysis(RDA)suggested that temperature was the main environmental factor affecting the community structure of phytoplankton, and it had a significant positive correlation with the density of phytoplankton. Water quality analysis indicated that the water in the net-pen closure area and lake area was all meso-eutrophicated, and there was no significant difference among the 3 groups.