Sterile seedlings of Potamogeton crispus L. were cultured by tissue culture technique, and were used as experimental materials. In this study, the effects of exogenous calcium (Ca) application on mercury (Hg) accumulation, mineral nutrition, photosynthetic pigments, superoxide anion, hydrogen peroxide, malondialdehyde (MDA), vitamin C and E, glutathione (GSH) and polyamines metabolism of the tested plants under the stress of 1.5 μmol/L Hg stress were investigated. The results demonstrated that, (1) Hg stress induced a substantial accumulation of Hg at 165.79 μg/g(FW) and resulted in serious imbalance of nutrient elements, such as inhibiting the absorption of P, K, Fe, Mg and Na, whereas exogenous Ca significantly reduced the accumulation of Hg by 66.4% and effectively reduced the Hg-induced imbalance of nutrient elements in Potamogeton crispus L.. (2) Hg treatment severely reduced chlorophyll concentration and chlorophyll-a/b ratio, and raised O₂·-generation rate, H₂O₂ and MDA contents, while increased the levels of V_C, V_E and GSH. Application of exogenous Ca markedly retarded the degradation of photosynthetic pigments and also significantly improved the levels of antioxidant capacity in the tested plants. (3) Hg treatment enhanced polyamine oxidase, diamine oxidase and arginine decarboxylase activities. Furthermore, reduced ornithine decarboxylase resulted in the decrease of putrescine (Put) and spermine (Spm) and in, slightly, the increase of spermidine (Spd) content. Exogenous Ca altered polyamines content of Hg stress by modulating polyamines metabolizing enzymes activities, mainly presented as significant increase in Put and Spm contents, and further increased the ratio of free (Spd + Spm)/Put. These results indicated that the supplement of exogenous Ca relieved the phytotoxicity of Hg to Potamogeton crispus L. by depressing the accumulation of Hg and involving in regulation of a series of physiological and biochemical mechanisms, particularly polyamines metabolism.