This study attempts to explore the physiological responses of Peridiniopsis bloom to phosphorus concentrations in Xiangxi River Bay of Three Gorges Reservoir.The population dynamics of phytoplankton and some environmental factors of waterbody were investigated on 9 sampling sites in the Xiangxi River Bay during water bloom period in spring of 2015.A strain of Peridiniopsis was isolated and purified from Xiangxi River.Specific growth rates of Peridiniopsis were measured at 5 different concentrations (0,0.005,0.020,0.100 and 0.600 mg/L) of phosphorus culture medium.Rapid light curves,photosynthetic oxygen evolution vs.in-organic carbon concentration curves and their parameters were examined at logarithmic growth phase.Results show that the high cell density of Peridiniopsis may indicate a high total phosphorus concentration.The conditions of high phosphorus could enhance photo-synthetic efficiency of Peridiniopsis,stimulate its growth,and increase its affinity for inorganic carbon.Therefore,high phosphorus content in water environment may be one of the key factors to promote the outbreak of dinoflagellate Peridiniopsis blooms.