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The aim of this study was to determine the key factors regulating the spatiotemporal variations in abundance of dominant phytoplankton in Shihmen Reservoir over an observation time of 5-years (2006一2010 ) .A dimension-reduction technique,dynamic factor analysis ( DFA ),was used to identify the underlying latent effects among multiple time series of phytoplankton abundance and the interactions between environmental variables and the phytoplankton abundance at five studied sites.The phytoplankton community was mainly composed of three dominant groups,namely dinoflagellates,diatoms,and green algae accounting for at least 90％ of all classes.The optimal DFA model successfully described the major phytoplankton abundance dynamics in Shihmen Reservoir.0ur results demonstrated that water level,water temperature,zooplankton. micro-zooplankton,nutrients(NH3-N,N03-N,and total phosphorus),and electrical conductivity were the key factors controlled the dynamics of dominant phytoplankton in this reservoir.NH3-N,the indicator of organic pollution,was the most important variable affecting the abundance of dominant phytophutkton.The distribution of phytophlankton in the reservoir exhibited site-specificity,showing a close correlation with the water quality at the inlet from the watershed.0ur results provide important basis for proposing management strategies to improve water quality as well as to control the degrees of eutrophication in this reservoir.