Impacts of Elodea Nattalii on water quality of grass carp pen culture area were studied in man-made elodea-grass-carp compound ecosystem. The result shows that water in plantation was better than that in area of non-macrophyte. Content of TN and TP in water were not increased because of purification of Elodea in planted area. By estimated balance of nitrogen and phosphorus in plantation, the appropriate of pen culture area to planted elodea area is 1:3.58. In addition, it was only partitive that breeded fish effected water quality in weeds-filled water body. So far as whole area of planted, there was little influence on water quality.