Based on the comparative studies between the enclosure culture situations of different density and scale of water environments investigated during period from March and December 2007 in East Lake Taihu, the paper analyzed the economical benefit and environmental effect under different culture models. The results showed that culture models of bait casting and monoculture in East Lake Taihu, no matter which kinds of density and scale there were, would make the output of nitrogen and phosphorus nutrients less than the input, and mass-input of exogenous nutrients would cause pollution effect on water quality. An estimation of nutrient contributions in three enclosure crab culture areas A, B and C showed that 1kg crab production has created 0.24, 0.33 and 0.30kg nitrogen and 0.043, 0.059 and 0.051kg phosphorus into the lake, respectively. According to the comparisons of environmental situations of economic benefit and growth specification of the enclosure crab culture in East Lake Taihu, a reasonable density of the optimized model is 6000 crab/ha and the single enclosure is 2.33 ha/each, while the current culture manner should be improved. For too large scale and unreasonable distribution of enclosure culture in East Lake Taihu, we suggest that the area of enclosure crab culture should be controlled under 3165.2 ha.