The study on the characteristics of sediment pollution and the boundary layer of water-soil interface is the key to identify the problem of sediment pollution under the long-term operation of large constructed wetlands. This paper takes a typical surface flow constructed wetland as an example, through comprehensive vertical layered monitoring of the physicochemical properties of the wetland sediment, to clarify the pollution characteristics of the sediment and analyzed the phenomenon, nature and formation mechanism of the boundary oxidation layer of the water-soil interface. The results show that, similar to lakes, the sediment of constructed wetland in long-term operation also generates oxidation layer, which is divided into oxidation layer, pollution layer, transition layer and health layer from top to bottom. After 6 years of operation, the wetland sediment can form an oxidation layer, which is about 1 cm thick, brown-yellow, malleable, with no obvious odor. Compared with the pollution layer, the color and smell of the oxidation layer are weakened, and its fluidity is weakened. Meanwhile, the water content, d(0.9), diatomic iron, total organic carbon and total nitrogen are decreased by 7.20%, 54.04%, 54.59%, 17.89% and 7.00%. Eh and total phosphorus are increased by 150.41% and 18.17%, respectively. Oxidation layer is a micro-environment of surface oxidation state formed under the synergistic action of oxidation and settlement. The higher the dissolved oxygen level of overlying water is, the more favorable it is to increase the density of the oxidation layer and reduce the organic matter and total nitrogen content of the oxidation layer. In addition, the higher the concentration of overlying water suspended substance and the deeper the water depth, the more favorable it is to increase the thickness of the oxidation layer.