Field control experiment and laboratory culture experiment were conducted to study the influence of nitrogen input on the freshwater mire carbon stock. The results shown that nitrogen input can increase bioaccumulation of carbon in the swamp, however, too much nitrogen input will reduce plant growth rate and prevent organic matter decomposition of flooding mire. The decomposition rate of organic matter in submersed is obviously larger than that of organic matter unsubmersed after input of nitrogen, suggesting hydrological condition and nitrogen input were both affected organic matter decomposition. After nitrogen input, carbon dioxide emission was significantly increased. However, too much nitrogen input induced decrease in carbon dioxide emission. Nitrogen input can stimulate plant growth and activity of soil microbe. On the other hand, too much nitrogen input could prevent plant growth and activity of soil microbe. Nitrogen input also affected obviously the concentration of dissolved organic carbon (DOC), and DOC concentration significantly decreased after nitrogen input ( P<0.05).