Abstract: The environmental problems caused by the Three Gorges Dam extremely is worth paying attention. A part of GaolanRiver became reservoir area after storing water. On 22 February 2005,a distinctive“algal bloom”was observed in Gaolan Riverand continued to March. The specie of forming“algal bloom”was a sort of Peridiniopsis sp. The density of Peridiniopsis sp andchl. a concentration reached 214 ×107cells/L and 33.73mg/m3 respectively during blooming. The algal bloom lasted about onemonth. In this paper the processes of the algal bloom and the basic phytoplankton composition were studied,and the functions ofthe environmental factors in the processes of the algal bloom in area scale were analysed. The environmental factors included water temperature,ammonium(NH4),nitrate (NO3),total nitrogen (TN),total phosphorus (TP),phosphate (PO4),dissolved oxygen(DO),CODmn,silicate (SiO2) and chlorophylla (chl. a). The studies revealed:High concentration of TN and TP ect. in the surfacelayer were likely to be one of the factors supporting the“algal bloom”in Gaolan Rive and suitable natural conditions also contributed to the process. When the nutritions in the upper layer were consumed away gradually by Peridiniopsis sp and finally wereexhausted for the development of bloom,a port of Peridiniopsis sp Sank into the bottom layer.