Abstract: Constructed wetland is a push-flowing bio-reactor with great ability of effluent purification. The structure of constructed wetland and the mechanism of effluent purification in pig farm is demonstrated. It has been shown that the component structure of the wetlands and the effect on effluent purification would be changing during the process from the beginning of operation to maturity. It has been discussed how to select the proper plant-seeds for the constructed wetlands, and "YaShe" is the best one selected It has been demonstrated that all of these index measured were reduced, including BOD₅, COD_Cr, SS, S^2- and Cu²⁺ reduced respectively 88. 04%, 88. 56%, 90. 77% , 88. 29% and 95. 74%. It is an ideal bio-technology to be used in the treatment system of effluent in pig farm owing to its high quality of water output and its convenient manner in management.