Limnoithona sineseis is considered as an endemic species in rivers of China with strong invasive ability. It was introduced to some reservoirs by pumping water and becomes the dominant species in the new water bodies. To understand the population dynamics of Limnoithona sineseis and its effects on the pelagic crustaceans, we investigated Dajingshan reservoir, a typical pumped storage reservoir located in Zhuhai City, South China in 2005 and 2006. In total, 13 species of crustaceans including 9 copepods(4 family 7 genera) and 4 cladocerans (3 family 4 genera) were identified. Limnoithona sineseis was dominant over the sampling periods. The abundance of the crustaceans ranged from 3.82ind./L to 364.85ind./L, with high abundance in spring. Limnoithona sineseis and its nauplius covered more than 50% of the total abundance. The body size of crustacean community in the reservoir had a range from 0.07mm to 1.25mm, and small-size species (within 0.07-0.4mm) was dominant. Population of Limnoithona sineseis was mainly composed of stageI nauplii, copepodite stage and adults. Nauplii had a much higher abundance than copepodites and adults. The abundance of Limnoithona sineseis was positively related to chlorophyll-a concentration, and negatively related to the pumped water volume. It meant that food was the principle factor affecting the animal population, and the animal had adapted well to its new habitat with relatively unconstant salinity, and its abundance was reduced by flushing of pumped water. As an invasive species, Limnoithona sineseis had become the dominant species by replacing the former ones (Mesocyclops), and its high abundance reduced biodiversity of crustaceans.