Pumped storage reservoirs are a type of water bodies which usually have small catchments and mostly store water that waspumped from near rivers. The special reservoirs play an important role in drinking water supply in coastal regions. In order to un-derstand eutrophication processes of these reservoirs, we investigated the Dajingshan Reservoir, a typical pumped storage reservoir,located in Zhuhai, Guangdong, a subtropical area of China. The reservoir was monitored and sampled every two times a month in2005. The monitored variables included nutrients(nitrogen and phosphorus),chlorophyll-a concentration, transparency, tempera-cure, precipitation and pumped water amount. The Irophic state index(TSI_M)of the reservoir ranged from 45 to 53,indicatingthat the reservoir was in meso-eutrophic states, but eutrophic state was present in the reservoir in most time of the year. There wasa seasonal variation of eutrophication with two peaks of chlorophyll-a concentration and TS/M in early spring and late autumn. Thisseasonal pattern is markedly different from those eutrophic lakes with only a peak in summer in temperate water body. Pumping we-ter into reservoir not only increased nutrient loading, but also modified largely hydrodynamic processes. Combining with the heavyprecipitation in summer, pumping water became a major factor affecting eutrophication processes, changing the pbytoplankton com-munity into a direct response to nutrient loading. The chlorophyll-a concentration correlations with total phosphorus concentration orwith total nitrogen concentration were markedly weakened, and the timing configuration of pumping water and precipitation have de-termined the ratio of chlorophyll-a concentration and nutrient concentration.