Wetlands have unique biogeochemical cycles with many chemical transformations and chemical transport processes that are not shared饰many other ecosystems. The construction of irrigation ditches in wetland changes the hydrology state, and thus influences the biological geochemistry circulation. In order to understand the effects of drainage ditches on the biogeochemical cycling in wetland, this paper takes the Sanjiang Plain as an example, and makes a detail introduction to the impacts of drainage ditches on the characteristics of movement, transformation, inputs and outputs of chemical elements and on the chemical mass balance. Results show that a lot of chemical elements are lost because of draining, which causes the degeneration of wetland.