This study focused on the changes in chlorophyll content, soluble protein content, O₂ ^.- generating rate, MDA content and activity of protective enzymes (SOD, CAT, POD) in leaves of Nymphoides peltatum under different concentrations of Cd²⁺ stress with 0.01 mmol.L^-1 exogenous spermidine. Transmission electron microscopy revealed that Cd²⁺ imposed injury on the ultrastructure of chloroplasts, with swelling, breakage and disappearance of the chloroplast envelope and reduced chlorophyll content. Spermidine treatment effectively protected the ultrastructure of chloroplasts against Cd²⁺ stress and maintained the arrangement of thylakoids. Under single Cd²⁺ stress, with Cd²⁺ increase, chlorophyll content in N. peltatum increased then decreased quickly, whereas soluble protein content decreased gradually. The content was much higher with the addition of sprayed spermidine than with only a single Cd²⁺ treatment, and the trend of decrease was lower. Under single Cd²⁺ stress, the maximal SOD, CAT and POD content appeared at 1, 1 and 2 mg.L^-1 Cd^2+_, respectively, and decreased gradually thereafter ; the activities of these protective enzymes were markedly increased under spermidine treatment, by 5.8%, 37.5%, and 3.3%, respectively, and therefore decreased the generation rate of reactive oxygen species such as O₂ ^-. and MDA content.