Nitrogen (4, 10, 20 and 40g/m2) and phosphorus (1.2, 4.8 and 9.6g/m2) were applied to tanks to evaluating the effects of N and P additions on plant tissue nutrition and the biomass of two freshwater wetland plants in the Sanjiang Plain of Northeast China, namely Carex lasiocarpa and Carex meyeriana. For C. lasiocarpa, the total N concentration (TN) of plant tissues under the treatment of 10g/m2 was lower compared with the other N treatments. Initially, C.lasiocarpa exhibited a significant increase of biomass as compared with the control value, reaching the maximum of 31.20±4.01 g/tank under the treatment of 10g/m2, and then dropped to 18.02±1.53g/tank under the treatment of 40g/m2.For C. meyeriana, TN generally increased with increasing amount of N applied. High N applied produced more aboveground biomass than low N applied. C. meyeriana, as the accompanying species, can adapt itself to the wetland enriched by N, and it may replace C. lasiocarpa as the dominant species of wetland. The total P concentration (TP) in tissues of C. lasiocarpa increased with P addition. The aboveground biomass of C. lasiocarpa increased with P addition,and it changed from 18.77±3.29g/tank to 46.03±3.95g/tank. However, TP of tissue may accelerate the C. meyeriana development under the treatment of 1.2g/m2. P accumulation contributes to the dominance of C. lasiocarpa but limits the production of C. meyeriana, and the latter may disappear gradually from the wetland enriched by P. Increased input of N and P might have an influence on wetland plant community composition and structure, so the effects of nutrient inputs and accumulation should be considered to protect the freshwater wetland.