Abstract: SWAT (Soil and Water Assessment Tool) model is a river basin, or watershed, scale model developed to predict the impact of land management practices on water, sediment, and agricultural chemical yields in large, complex watersheds with varying soils, land use, and management conditions over long periods of timeThe model is physically based and enables users to study long term impacts This paper mainly introduces and discusses the issues of the hydrological theory, the structure and functions of SWAT modelSWAT separates the hydrology of a watershed into two major divisionsThe first division is the land phase of the hydrologic cycle which controls the loadings to the main channel in each subbasinThe second division is the water or routing phase of the hydrologic cycle which can be defined as the movement of water, sediments, etcthrough the channel network of the watershed to the outletSWAT uses a command structure for routing runoff and chemicals through a watershedUsing a routing command language, the model can simulate a basin subdivided into grid cells or subbasin In case study, SWAT model was used to simulate the hydrology of Heihe (Yingluo Valley) Basin in the cold Northwest ChinaFirst, Based on DEM, Heihe (Yingluo Valley) Basin was subdivided into four subbasinsBy building user soil type database and modifying land use coding, the model made a good runoff simulation result on a daily time step,and the model NSE (Nash Sutcliffe error judge standard) is up to 083So.

Key words: Application Study, Distributed hydrological model, Hehei basin, SWAT