In pumped storage reservoir, the phytoplankton community is regulated in a distinct way from that in the other types of reservoirs. To study its dynamics, we biweekly sampled the phytoplankton in Dajinshan Reservoir in 2005. Dajinshan Reservoir, is in the northern part of the in tropics in Guangdong Province. It is a typical pumped storage reservoir. 100 taxa were identified. Phytoplankton had an abundance ranging from 0.86×106to 106.27×10⁶cells•L^-1, and total biomass varied from 1.08 to 20.45 mg•L^-1. The phytoplankton community was dominated by Pseudanabaena sp. across all the sampling year. Synedra ulna. had a high biomass in spring only. The composition of phytoplankton community was highly stable. Canonical correspondence analysis was used to investigate the environment-phytoplankton relationship. Case ordination by CCA divided all samples into four groups, distributed in the four districts forming axis 1 and axis 2, corresponding to the seasons: winter, spring, summer and autumn. Most samples were located in the districts of summer and autumn，and 54 main species were selected for performing of a CCA. The Cyanophyta, Bacillariophyta and Euglenophyta were restricted to the districts at the left of axis 1, While most of the Chlorophyta, Chrysophyta, and Cryptophyta were at the right of axis 1. Temperature and precipitation were the most important factors regulating the composition and dynamics of the community.