Abstract: Along with the reclamation of Sanjiang plain mire-wetland area,the paddy field's drainage goes into the natural freshwater of the mire-wetland,which will influence the wetland's eco-systems ultimately. When paddy field drainage with certain concentrations of nitrogen and phosphorus are discharged into the Carex lasiocarpa mire-wetland water,the concentrations of TN,NH₄⁺ N,TP and PO₄^3- P all increase obviously. Especially in August and September the concentrations of TN and NH₄⁺ N are 1.51-2.10 times and 1.53-3.02 times as much as those of natural mire-wetland freshwater respectively,and the concentrations of TP and PO₄^3- P are 1.30-4.08 times and 4.33-11.33 times as much as those of natural mire-wetland freshwater respectively. The biomass of the roots and stem-leaves of Carex lasiocarpa increases in the wetland with the paddy field drainage compared with those in natural wetland. The contents of nitrogen and phosphorus in the roots and stem-leaves of Carex lasiocarpa in the wetland with the paddy field drainage are much higher than those in natural wetland,and the correlation coefficients between TN and TP concentrations in Carex lasiocarpa roots and wetland water TN and TP contents are higher in the wetland accepting the paddy field drainage,which indicates that the paddy field drainage can improve Carex lasiocarp a growth and the absorption of nitrogen and phosphorus. Therefore,Paddy field drainage should be prohibited from discharging into wetland directly because of the unbalance of N/P otherwise it will threaten the stability and bio-productivity of wetland eco-systems.