Abstract: The Xijiang Oxbow in Anqing City belongs to the old course of the Yangtze River. It is the third ex situ conservation zone for Yangtze finless porpoise (YFP, Neophocaena asiaeorientalis asiaeorientalis) in China. To understand the community structure of zooplankton and assess the survivability of the YFP, a field investigation of seasonal variation of zooplankton and environmental parameters was conducted in Xijiang Oxbow from April 2015 to January 2016. The results showed that: (1) a total of 55 species were identified, and the number of species belonging to Protozoa, Rotifera, Cladocera and Copepoda was 13, 27, 9 and 6, respectively. The number of zooplankton species varied greatly between seasons, with the most number during summer (39 species) and the least number during winter (16 species). (2) The annual average density and biomass of zooplankton in Xijiang Oxbow were 4115 ind./L and 1.735 mg/L respectively, both of which were dominated by Protozoa and Rotifera. One-way ANOVA analysis showed that the density and biomass of zooplankton were significantly higher in summer compared with other seasons. (3) A total of 12 main species were recorded throughout the whole year, including 4 Protozoa, 6 Rotifera, and 2 Copepoda. The dominant species composition differed between seasons. (4) The assessment results of water quality showed that the Xijiang Oxbow was classified as mesotrophic level considering the biomass of zooplankton, and α-mesosaprobic using Shannon-Weiner species diversity and Margalef species richness index. It was indicated that the water quality satisfied the living requirement for YFP. (5) Based on the standing crop of zooplankton, it was estimated that the fish productivity in Xijiang Oxbow is about 54340.2 kg, which could meet the nutrition requirements of 36 YFPs. The results of this study not only improved the understanding of the community structure of zooplankton in Oxbow waters, but also have important implication to the protection of YFP in ex situ reserve.