Dense algal blooms in the Lake Taihu have become known worldwide.The goal of the present study was to assess the growth and species responses of freshwater algae to variation in water quality in Qinshan Lake,which is located in headwaters of the Taihu watershed.Water samples were collected monthly from ten sites of Qinshan Lake and were analyzed for species distribution of freshwater algae and physichemicai parameters such as TN、NH4+ - N、NO3- - N、TP、CODMn、Chl - a.The results showed that average TN was 4.47 mg/L,with 92.2％ of values greater than the TN standard ; average TP was 0.051 mg/L,with 37.9％ of values greater than the TP standard; and average Trophic Level Index (TLI) of Qinshan Lake was 53,which indicates the low beginning of the range of eutrophic conditions.Average Chl - a concentration in Qinshan Lake was 12.83 mg/m3.Green algae and diatom far outweighed other freshwater algae and dominated during most of the year,with the highest relative abundances of which was 96％ and 99％,respectively.Blue - green algae became most dominate in the summer with maximum relative abundance of 69％,composed mainly toxic strains like Microcystis sp.,Nestoc sp.and Oscillatoria sp.Nutrients were the main restrictive factor of population growth of blue - green algae,especially control ofammonia and covarying nutrient loads was the key problem for the prevention of algal blooms in Qinshan Lake.