Abstract: In an experiment, Nymphoides peltatum (Gmel) O.Kuntze, an aquatic plant, was cultured in solution with varying concentration of CdCl₂ for a varying period of time. Then samples of its stems and leaves, roots, and the culture solution were collected and determined for content of Cd ²⁺ ,O ₂-· and MDA in the plant cell, activity of SOD and exosmosis. The results show that the plant is fairly tolerable to Cd ²⁺ with 5mg/L being its minimum lethal concentration. It purifies Cd ²⁺ containing sewage by roots absorbing and exudate adsorbing Cd ²⁺ in the solution, thus removing about 99.5% of the Cd ²⁺in the solution. Within the range of tolerable Cd ²⁺ concentration, the plant has higher SOD activity to balance the large amount of O ₂-· generated as a result of Cd ²⁺ poisoning, thus effectively prohibiting the formation of MDA. This is believed to be one of its defensive mechanism under Cd ²⁺ stress.