To investigate the composition and seasonal dynamic of the zooplankton communities in tropical reservoirs, environmental variables and zooplankton communities in the pelagic zone of Dashahe Reservoir (Guangdong, South China) were monthly investigated from January of 2012 to December of 2013. In total, 44 species of zooplankton were recorded, including 5 in Copepoda, 9 in Cladocera and 30 in Rotifera. The species composition of zooplankton showed a weak seasonal change, due to many common species between seasons. The fluctuation range of the zooplankton total biomass in 2012 and 2013 were 210.1-2560.5 μg/L and 512.7-3145.6 μg/L, respectively. Cladocera dominated the zooplankton in biomass and accounted for 45.5% in 2012 and 51.5% in 2013. Copepoda represented 31% and 40% of the total biomass in 2012 and 2013, respectively. Cladocera was not significantly correlated with Rotifera in abundance and biomass, indicating a low competition relationship. The total biomass of zooplankton in two years had no significant difference, but the community structures were quite different. The averaged community distance (Bray-Curtis) of month pairs between 2012 and 2013 was low (0.61 with a standard deviation of 0.18) indicated a strong randomness in dynamic of community structure. The total abundance and biomass of zooplankton showed a significant seasonality just following hydrological dynamics, and were higher in the dry period. Redundancy analysis of the zooplankton community and environmental variables demonstrated that food abundance did not explain much of the variation of the zooplankton community, but temperature and water transparency caused by hydrological changed are strongly related to the variation and randomness of zooplankton community composition.