Lake Honghu, crossing over Honghu county and Jianli county in Hubei Province, China, is situated in the north of the middle reaches of Yangtze River. Because of unreasonable exploitation of wetland resources and discharge of pollutants,it confronted with series problems such as decrease of the lake area and wetland biodiversity, sharp reduction of rare species and its eutrophication of water body. Study for planktonic algae reflecting water ecological environment change will provide a scientific basis for protection of Lake Honghu. Algae are important composition of aquatic ecosystem; their community structure and function are adaptable to water environment, and vary with changing water environment. The characteristics of community structure can be used as an important index in the assessment of water quality. In order to understand the change of water environment in Lake Honghu, Hubei, China, we measured a set of variables related to water quality, including structure and density of algous community, water level(H),water temperature (t_w), transparency(SD), permanganate index (COD_Mn), total nitrogen (TN), total phosphorus (TP) and chlorophyll a content(Chl-a). The above experiments were conducted monthly between 2009 and 2010 at the five sections in Lake Honghu. The nutrition state was evaluated with eutrophication index for Lake and Reservoir; the influence of some environmental factors on algous cell density was analyzed by correlation analysis and stepwise multiple regression; and some stepwise multiple regression equations were established. Analytical results showed that the water was in light eutrophication in Lake Honghu. Totally, 7 phylum and 65 genera of planktonic algae were identified in the lake. The algous cell density ranged from 1.14 million cells per litre water to 32.4 million per litre water. Bacillariophyta was the most abundant phylum of planktonic algae, the following were Chlorophyta and Cyanophyta, in view of algous population distribution. Its algous community are 52.93% for Cyanophyta, 25.96% for Bacillariophyta, 16.83% for Cyanobacteria, 1.98% for Cryptophyta, 1.64% for Chrysophyta, 0.42% for Euglenophyta, and 0.25% for Pyrrophyta. In light of structure and density of algous community change seasonally, the dominant species are Melosira and Cyclotena within Bacillariophyta in winter and spring, Anabaena of Chlorophyta in summer and autumn. There is a statistically significant positive relationship between algous cell density, water temperature and permanganate index, whereas there is no obvious relationship between algous cell density, water level, total phosphorus, total nitrogen and transparency. The Stepwise regression equation of growth for planktonic algae was that: y=3.7815+0.0794t_w+0.5670COD_Mn-1.3467SD (R²=0.618, F=8.087, P<0.02). This indicates that algous cell density could be used to reflect the pollution level caused by water temperature and organic pollutants, and the controlling of nitrogen and phosphorus has not been tone of the key factors. The characteristics of algous cell density are controlled by the different environmental conditions at the five sections, such as Bacillariophyta controlled by water temperature, permanganate index and transparency, Chlorophyta controlled by water level (H), water temperature and permanganate index, and Cyanophyta controlled by permanganate index. Based on the results, there are great significance for the effective measures on slowdown of water eutrophication and inhibit of the algous growth process in Lake Honghu, by controlling nitrogen, phosphorus and organic pollutants inputs of Sihu Main Cannal, reducing cages culture area, and restoring water connectivity of Yangtze River and Lake Honghu.