During the passed decade, in shallow lakes of the middle and down stream basin of the Yangtze River, especially in urban lakes, there has been an explosive development of pen fish farming, and a rapid increase in fish yield and economic benefit. However, due to the input of artificial food, over-exploitation of aquatic higher plants and increase in population, submerged vegetation have decreased and even disappeared, the process of eutrophication has been quickened and the total function of lakes has been weakened. Elodea Nuttallii was introduced into the East Taihu Lake from abroad in 1986, and soon became established. It has been used as food of cultivated herbiverous fishes by the local fishermen. Based on the biological and ecological property of Elodea, in this project we studied:(1) the planting method, cutting time and mode of Elodea in shallow lakes in order to increase the yield per unit area; (2) the digestibility and food conversion efficiency of Elodea by grass carp so as to determine the ratio of fish yield to Elodea planting area; (3) the role of Elodea in the ecosystem. Finally, we designed and established an artificial compound ecosystem of Eiodea-grass carp pen farming, i. e., on the basis of planting Elodea which is used for grass carp culture and functions as nutrient cycling medium, the grass-fish balance system is established within large lake ecosystems, which should lead to a better development of lakes.