Digital Elevation Model DEM has been widely used in hydrological models because of its ability to automatically recognize the drainage structure information required in watershed hydrological models.Some questions on the implement of DEM in watershed hydrological simulation are discussed including the assignment of the drainage directions over a DEM the recognition of the drainage network and the treatment of pits.A suitable data structure of DEM and scale of grid are presented also.How to modify the modeled watershed drainage network is discussed.