Four investigations were conducted on the zooplankton in Daya Bay, in March, May, September, and December 2004. A total of 128 zooplankton species and 14 pelagic larvae were identified. Zooplankton appeared in summer (90 species), autumn (81 species), winter (71 species), and spring (47 species). The abundance of Zooplankton was 101338 ind?m-3 (summer), 913.30 ind?m-3 (autumn), 437.86 ind?m-3 (winter), and 162.37 ind?m-3 (spring). The seasonal variation of Zooplankton biomass was 773.89 mg?m-3 in autumn, 472.82 mg?m-3 in Summer, 286.44 mg?m-3 in winter, and 164.11 mg?m-3 in spring. The level of biomass was close to the species composition，the dominant species in the autumn were large zooplankton, such as Jellyfish and Chaetognatha. Therefore, the biomass was high. The trend of the H′,J and D varitation was close for the four investigations. The three indexes were the lowest in the spring, the highest in winter, summer, autumn more or less the same. In addition, results of the analysis showed that Chaetognatha, which was a dominant species, appeared in four seasons, indicating the response to rising water temperature. Zooplankton biomass in the coast was higher than that in the central of the Bay. According to the abundance distribution, the environment of the zooplankton in the Daya Bay had been affected by large-scale construction projects and human activities.