Elodea-grass carp pen farming is a relatively independent subsystem artificially established in the lake, in which the grass carp feeds on Elodea Nattallii, the excretory matters of fish transform into nutrient salts, and nutrient salts transform into the circulatory system of Elodea. The transformed organic matters are moved out of the system through fish and there are no extra organic matters remained in the system, so fish-farming environment is optimized. Based on the above-mentioned, we selected East Taihu lake as the experiment base and established a semi-closed experiment region, in which there are two fish-farming districts, Elodea-growing district and no-weed district. The changes of bacteria, phytoplankton, zooplankton, zoobenthos and organic detritus have been analysed. The results show:(1) The density of heterotrophic bacteria decreases gradually from fish-farming district to outside, decrease 88.9% in Elodea-growing district and 73.9% in no-weed district. The amount of coliform group is similar to heterotrophic bacteria, decreases 75.3% and 66.7% respectively in both districts. (2) The average density and biomass of phytoplankton are 4 612.187×10⁴ ind./L and 7.1776mg/L in Elodea-growing district and 6070.853×10⁴ ind./L and 8.3108mg/L in no-weed district respentively.(3) 92 and 60 species of zooplankton (including Rotifera, Cladocera and Copepoda) have been found in Eiodea-gvoWmg and no-weed district, respectively. The average density and biomass are 4 752 ind./L and 4.868mg/L and 6 749 ind./Land 3.9407mg/L respectively. (4) 19 and 14 species of zoobenthos (including Annelida, Mollusca and Arthropoda) have been found respectively in the two districts. Although the densities are the same in two districts, the compositions of community and biomass are different. The average density and biomass of Annelida (mainly oligochaete) are 59 ind./m² and 0.575mg/m², 73 ind./m² and L 453mg/m²; Mollusca are 80 ind./m² and 34.822mg/m², 29 ind./m² and 12.065mg/m²; Arthropoda(mainly chironomid larvae) are 78 ind./m² and 0.673mg/m², 115 ind./m² and 0.688mg/m² in two districts respectively. (5) According to the average depositional rate of organic matters, it is as 5 times in no-weed district as that in Elodea-growing district. Due to the macrophyte, the buffer capacity effected by environment is strengthened in the Eoldea-growing district. Not only is the water body purified and the water quality recovered. but also the community structure of hydrobios is more stable and species diversity is increased. Eutrophication is also controlled effectively. So, Elodea-grass carp pen farming is a optimum way of eco-fishery development.