In order to analyze the community structure of zooplankton and its influence factors in Siminghu Reservoir, the species and abundance of zooplankton and the physical-chemical factors were tracked by month from August 2012 to July 2013. 150 species of zooplankton were identified in Siminghu reservoirs, and the ranges of cell abundance and biomass were 198.0﹣7 258.0 ind/L and 0.28﹣5.64 mg/L, respectively. Analysis of variance showed that both of the abundance and biomass had a significant seasonal variation but no spatial difference, as was showed in Fig. 1 and Fig. 2. Correspondence analysis was used to demonstrate the relationship between the zooplankton community and the physical-chemical factors. The result, as was displayed in Table 2 and Fig. 4, showed that water temperature, chlorophyll a, total phosphorus, euphotic layer, ammonia nitrogen and water level were the dominant factors affecting zooplankton community structure. Spatial differences existed in the dominant factors between the sample points. A decrease in the abundance and biomass and miniaturization in the size of zooplankton were found by comparing with the date from 24 years ago, which possibly resulting from top-down effect by fish predation and zooplankton fertility decline caused by the increase of cyanobacteria density and microcystin. Water quality evaluation results show that the reservoir is at a mesotrophic stage now, but has the trend of increasing to eutrophic stage.