An experiment with treatments differing in nitrogen supply(0,5and 15g/m²)and CO₂ level(360and 700μmol/mol)with OTC(open top chamber)equipment was carried out to investigate the effects of elevated CO₂concentration on the carbon and nitrogen accumulation of Calamagrostis angustifoliain freshwater marsh of the Sanjiang Plain.Results showed that elevated CO₂concentration increased carbon fixation ofplant by 19.3%,24.4% and 24.6% under the NN,MN and HN treatments,respectively.Elevated CO₂concentration also had a great impact on the carbon allocation in different parts of Calamagrostis angustifolia,as the carbon distribution in root increased to different degrees under all the nitrogen treatments.Elevated CO₂ concentration reduced the nitrogen content of Calamagrostisangustifoliaapparatus. The largest decrease in leaf and stem both appeared in heading period(14.4% and 19.5%),while the decrease in root appeared in dough period(17.4%).The decrease of nitrogen content in apparatus was due to diluting effect byplant speeding growth under elevated CO₂ concentration.