Because a lot of nitrogen and phosphorus flow into lakes and rivers, water bloom frequently occurs inworld scope. Recently a sori of Peridiniopsis sp. bloom was appeared in Lake Donghu, Wuhan. The Peridirtiopsisap. can adapt well to a large,,of temperature. The algal bloom lasted a long time and the maximum density ofPeridiniopsis ap.reached as high as 2. 54 x 10⁷ cells/L. In the paper, population dynamics of Peridiniopsis ap.andits relationship with environmental factors in Lake Donghu were studied. The correlation analysis indicated Peridini-opsis sp.be remarkable positive wrrelation with the concentrations of TP _surface,TN_1.2metermand N0₃-N _surface. However, the concentrations of TP, TOP, TN, TON, PO₄-P and NH₄-N were raised just a day before blooming and feVto lowest after blooming. Suitable strength of illumination may be a triggering factor for blooming. In addition, itcan be found from the composition and distribution of phytoplarJcton in some sampling stations that in certain degreewind can make Peridiniopsis ap. gather and cause bloom. The development, occurring and dying out of algal bloomwas observed. When the alga bloom died out, a lot of Peridiniopsis ap. sunk down toward lower layer. By analy-zing, authors consider many environmental factors lead to water bloom together.