Jiantan Reservoir was newly constructed in 2006,and it took eight months to fill the reservoir from October 2006 to June2007. The reservoir is a through-flowing reservoir with retention time varying from 0.46 to 3. 62 d in 2008. Phytoplankton in thereservoir, was investigated to analyze early variation of water quality from 2006 to 2008. Phytoplankton abundance varied from 182to 6267cells/ml, and was predominated by Chlorophyta. During the investigation, phytoplankton abundance generally increasedfrom the upstream to downstream. After impoundment, both phytoplankton abundance and the relative abundance of Cyanophytawere much higher than those before impoundment or during filling at the dam site. Water retention time, secchi disk depth andtotal phosphorus were the three possible factors controlling the temporal and spatial variations of phytoplankton abundance and com-munity structure. High advective loss rate and low secchi disk depth made the standing crop of phytoplankton being lower than thatin those reservoirs with the same trophic state but longer water retention time. In December, 2008,the first year after filling, therelatively longer water retention time, higher secchi disk depth and total phosphorus concentration resulted in the“upsurge" ofphytoplankton abundance.