Because a lot of nitrogen and phosphorus flow into lakes and rivers, water bloom frequently occurs in world scope. Recently a sort of Peridiniopsis sp. bloom was appeared in Lake Donghu, Wuhan. The Peridiniopsis sp. can adapt well to a large range of temperature. The algal bloom lasted a long time and the maximum density of Peridiniopsis sp. reached as high as 2.54 x 10(7) cells/L. In the paper, population on dynamics of Peridiniopsis sp. and its relationship with environmental factors in Lake Donghu were studied. The correlation analysis indicated Peridiniopsis sp. be remarkable positive correlation with the concentrations of TPsurface, TN1.2 meters and NO3-N-surface. However, the concentrations of TP, TOP, TN, TON, PO4-P and NH4-N were raised just a day before blooming and fell to lowest after blooming. Suitable strength of illumination may be a triggering factor for blooming. In addition, it can be found from the composition and distribution of phytoplankton in some sampling stations that in certain degree wind can make Peridiniopsis sp. gather and cause bloom. The development, occurring and dying out of algal bloom was observed. When the algal bloom died out, a lot of Peridiniopsis sp. sunk down toward lower layer. By analyzing, authors consider many environmental factors lead to water bloom together.