To reduce the pollution of water by cage culture of fish and explore the environmentally-friendly net cages, we took the Pistia stratiotes and Ceratophyllum demersum, the preponderant aquatic plants, as the objects of the research, with the intention of examining their effects on water purification in terms of cage culture by installing Ceratophyllum demersum-Pistia stratiotes mixotrophic ecological net cage. The conventional net cage was only covered with shade cloth without aquatic plants.The net cage for the experiment was 5 m×4 m×2.5 m; the width of the periphery cage was 1.5 m and its depth was 1 m. Ceratophyllum demersum was grown in the periphery cage and Pistia stratiotes L in the net cage. The results indicate that the recovery rates of nitrogen and phosphorus of ecological cage with Ceratophyllum demersum-Pistia stratiotes L. mixed culture were 49.86%±0.94% and 38.07%±0.62%, respectively, and the recovery rates of nitrogen and phosphorus of the traditional cage were 44.60%±0.85% and 33.17%±0.58%, respectively. For the collection rates of nitrogen and phosphorus, there was striking variation between ecological cage and traditional cage (P<0.05). The rates of usage of nitrogen and phosphorus of ecological cage with Ceratophyllum demersum-Pistia stratiotes mixed culture were 28.13%±1.48% and 16.95%±1.09%, respectively, and the utilization rates of nitrogen and phosphorus of the traditional cage were 26.40%±1.89% and 15.64%±1.47%, respectively. For the efficiency of nitrogen and phosphorus, there was no significant difference between ecological cage and traditional cage(P>0.05). Through analysis of the means and the total amount of input and output of nitrogen and phosphorus in cage fishing system, we concluded that there is zero emission of nitrogen and phosphorus in cages of culture of Lieocassis longiristris, if the ratio of the area of the cage to the area of culture of Pistia stratiotes and Ceratophyllum demersum is 1:17.00.