Lake Gehu is a representative shallow lake located in the downstream of Yangtze River in China. To identify the phytoplankton community distribution after its treatment, a survey was conducted on the phytoplanktonic community and environmental factors in its north part from January to December in 2013. According to the survey, there were altogether seven phylum, fourty-three genus and sixty-one species of phytoplankton, mainly with Scenedesmus and Cyclotella in spring and winter, Microcystis and Melosira granulata in summer and autumn. The maximum phytoplankton biomass during sampling period was 90.6mg/L in August, mainly with Microcystis aeruginosa. The average density of phytoplankton showed a decreasing trend from west to east, with lower vegetation coverage in the open water area. The environmental factor analysis indicated that total nitrogen, total phosphorus and water temperature were key factors that influence the phytoplanktonic density and biomass in this area. Comparing with the average biomass in the water chestnut area and reed area with that in the untreated open water area in the same month, we found that the water chestnut area decreased with the range of 72.7%-91.1%, and 63.9%-83.7% in the reed area. In August and September when algal bloom occurred in the open water area, the biomass in the chestnut area and in the reed area was only 14.6% and 30.3% of that in the water area, respectively.