| Water is an important physical foundation for people's existence and development. With the rapid development of economy and society, the pollution phenomenon of water is more and more serious. The quality of available water resource continuously decreased, and environment of water sustained deteriorated. Non-point pollutants from agricultural sources are major cause of surface water eutrophication, which can be trapped and removed by adsorption, sedimentation, assimilation, and microbe decomposition of wetlands. Due to lower concentration, large range and discharge ways of agriculture Non-point pollution, drain wetland play an important role and it is applied widely in agricultural Non-point pollution. So, studying on the mechanism of the drain wetland restore water environment in semi-arid has important theoretical meaning and practical values in control of agricultural Non-point pollution, restoration of water environment and sustained utilization.Removal mechanism of N and P in drain wetland in semi-arid area is discussed in this paper. It shows that N is migrated and transformed in processes of ammoniation, nitration, denitrification, nitrogen-fixation and assimilation of nitrogen, then it is denitrogenated in absorption of plant, volatilization, volatilization in medium and nitrification-denitrification of microorganism, P is lost in processes of surface runoff, then it is removed in absorption of plant, physico-chemical effect and removal of microorganisms.The hydrological process of drain wetland in semi-arid area is discussed; the mathematical model of drain wetland in semi-arid area is established in this paper. Then the computer program with SWMS2D model is compiled. The results from numerical simulation of years in different water serous level are show that drain lining works, ditch distance are important in hydrological process of the drain. It provides theoretical and analysis methods to the quantitative study of the drain wetland in semi-arid area.
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