CH₄ and N₂O are the two main greenhouse gases with great contribution to global warming. Natural wetland in the north hemisphere is recognized as one of the most important global sources of atmospheric CH₄. But its role in contribution to N₂O is not clear. Nutrient enrichment may impact the greenhouse gas emission from freshwater marshes. As the largest freshwater marsh distribution area in China, Saijiang Plain has experienced the extensive reclamation over the past 50 years. Large area of the marshes was drained for agricultural production. The remained marshes are often subject to high levels of nitrogen enrichment from atmospheric deposition, surface runoff loading and agricultural drainage, which affected the CH₄ and N₂O emission from marsh ecosystem. In this work, we discuss the effects of exogenous Nitrogen on the CH₄ and N₂O fluxes of marshes using an in situ field fertilization experiment with the static chamber technique. Ammonia Nitrate solution were applied at a rate of 0 (control), 60, 120kgN•hm^-2 and 240kgN•hm^-2. Monitoring results showed the maximum CH₄ flux of these four levels is 3.67mg CH₄ m^-2 h^-1 ,11.01 mg CH₄ m^-2 h^-1, 11.28 mg CH₄ m^-2 h^-1 and 5.82 mg CH₄ m^-2 h^-1, While the maximum N₂O flux is 0.27 mg •m^-2•h^-1, 0.65 mg•m^-2•h^-1, 1.0 mg•m^-2•h^-1, 2.26 mg•m^-2•h^-1. The results indicated exogenous nitrogen input increases the CH₄ and N₂O emission. Comparing with the control level, the CH₄ flux of other three fertilization levels increases by 181％, 254％and 155％, and the N₂O fluxes increased 21％, 100％and 533％, respectively. During the growing season, nitrogen input does not have obvious influence upon the seasonal dynamic pattern of CH₄ emission flux. But every Nitrogen input, especially of the highest level, brings about the obvious fluctuation of the N₂O emission flux. With the increasing of nitrogen input, there is an exponential increase trend of N₂O flux (R²＝0.97，p<0.01).CH₄ flux under the high nitrogen input was lower than those under the low and medial nitrogen input，which was related to the plant growing stages and the microbial activity of CH₄-producing. The exogenous nitrogen input changed the CH₄ and N₂O fluxes not only via the direct effect but also by affecting the wetland microbial process.