The crustacean plankton community structure and water quality were compared between freshwater prawn (Macrobrachium rosenbergi) monoculture ponds and prawn+pearl mussel (M. rosenbergi+Hyriopsis cumingii) polyculture ponds located in Baibu Town, Haiyan County, Zhejiang Province from June to October in 2011. This study aimed at understanding the ecological effects of prawn + pearl mussel polycuture. TN, TP, NO₃-N and DO were significantly lower in polyculture ponds than those in monoculture ponds, and no differences in other parameters of water chemistry between two culturing modes were found. A total of 22 crustacean plankton species were identified, including 13 Cladoceran species and 9 Copepod species, accounting for 59.09% and 49.91% of the total species number of crustacean plankton, respectively. Significant difference between culture modes in Shannon-wiener diversity index of the crustacean plankton was found, but not detected in Pielou evenness index. Density and biomass in monoculture ponds were significantly higher than those in polyculture ponds. Stepwise regression analysis showed that water temperature and total phosphorus from 12 environmental factors were the main factors affecting density and biomass of crustacean plankton. Canonical correspondence analysis (CCA) indicated that water temperature was the key factor influencing the temporal and spatial distribution of crustacean plankton. The output of M. rosenbergii (533.30±104.24 kg/m²) was significantly lower in monoculture ponds than that in polyculture ponds (866.70±214.92 kg/m²) (P<0.01). Results suggest that prawn and pearl mussel polyculture can improve the water quality of the aquaculture ponds. Although the polyculture can't significantly change the species composition of crustacean plankton, it can significantly decrease the diversity, density and biomass of crustacean plankton. Meanwhile, prawn and mussel polyculture can also raise the output of prawn.