Abstract: Treating the Seedling of Trapa bispinosa using different concentrations of Cd²⁺ and Hg²⁺ ,the effects of Cd²⁺ and Hg²⁺ on growth and activity of superoxide dismutase (SOD) and peroxidase (POD) in the species were studied.All treatments with different concentrations Cd²⁺ 、Hg²⁺ can inhibit growth and reduce chlorophyll content of T.bispinosa seedlings,but the inhibition of Cd²⁺ are more significant than that of Hg²⁺ .Effects of Cd²⁺ and Hg²⁺ on activity of SOD and POD are different:treatment with Cd²⁺ can initially induce the activity of SOD and POD to rise,but with extending time and increasing concentration,the activity declines sharply;while activity of SOD and POD treated with Hg²⁺ relatively changes smoothly,in which activity of POD treated with 5 μmol/L and 10 μmol/L Hg²⁺ rises continually.Experimental results indicate that toxicity of Cd²⁺ on T.bispinosa seedlings is heavier than that of Hg²⁺ under the same treating time and concentration.According to the results,authors try to inquire into the toxicity mechanism of heavy metal on plants.