Abstract: The paper represents the phenomenon of water blooms occurring in Three Gorges Reservoir from July 2003 to April2005. During this period, algal water blooms occurred for many times, especially in spring when the water temperature risedgradually and sunlight becomed abundant. The main taxa of water blooms were Peridiniopsis sp1, Peridiniopsis sp2, Stephanodis-cus neoastraea, Cyclotella caspia, Rhodomonas lacustris, Pandorina morum and Eudorina elegans, in which water bloom ofPeridiniopsis sp1 was the most serious one. When water-bloom of dominant specie Peridiniopsis sp1 emerged, Cyclotella sp.,Cryptomonas sp. and Melosira sp. supplied much primary productivity too. The peak values of cell density and biomass of Peridiniopsis sp1 were noted at 1 meter with cell numbers of 8171 ×106cells/L and the wet weight of 69165mg/L. The minimal valuesof two indexes were both noted at bottom layer with 4158 ×105cells/L and 3167mg/L. Other algae also represented their owndistribution orderliness. Contrasting cell densities and biomass of different algae, we educed that primary productivity was supplied by Peridiniopsis sp1 due to its great cell volume and huge cell numbers. For proving up whether Peridiniopsis sp1 waterbloom produced toxin and brought effects on human’s drinking water, we made out toxicological experiments on waterbloomwater and algal distilled solution. The water sample taken in Tongzhuang River was filtered through Whatman GF/C filters(0145μm) to remove natural particles and other organisms prior to experiment. We prepared seven testing medium, namely original water, original water + N, original water + P, original water + N + P, original water + full nutritional elements in BG11, autoclaved original water and BG11. The tested alga was Scenedesmus obliquus and the original cell density was 715 ×105 cells/L.The culture vessels was 150mL autoclaved glass flasks containing 100mL water. The flasks were incubated at 20 ℃with a lightintensity of 50-70μmol/m2/s. Bottom light was supplied at all times. The samples were cultured for sever days and the algalgrowth was examined everyday by light absorbance values (OD650). The experiment was repeated three times and results wereanalysed with percentage to contrast. The Peridiniopsis sp1 for mouce toxic bioassay was also taken in the Tongzhuang River with25 # phytoplankton net. Zooplankton and other impurities were removed by filtering and centrifugating. Algal distilled solutionwas obtained by freezing and skiving repeatedly, also centrifugating with 10000r/min at 4 ℃. The expeiment was performed in sixgroups and ead each group has three mice. Different concentration of algal distilled solution (1 、1/2 、1/4 、1/8 、1/16)was injected in mice’s abdomen. The control group was 019% NaCl solution. By observing mice’s feedback symptom and it’s weightchange, we judged whether mice were poisoned. According to the results about the toxic tests, the authors elicited primarily thatthe waterbloom of Peridiniopsis sp1 had no obvious biotoxicity to other aquatic organisms and small white mouse. Analysingmany factors affecting waterblooms in Three Gorges Reservoir, we considered that waterblooms were caused by industrial effluents, life sewage export, slow current speed and appropriate temperature and so on. In addition, wind changed distribution stateof waterbloom algae |It was that ship and water bird played a accelerated pole in algae movement. And also, temperature changeand raining affected kinds of waterbloom algae and time of waterbloom lasting. We put forward primarily two ways to preventwaterbloom in Three Gorges Reservoir, namely reducing nutrients input and increasing water velocity.