DEM resolution has important effects on process simulation of runoff and sediment. However, the internal mechanism ofthem are unclear. Through calculating the slope separately by the module instead of the gradient extraction directly by the DEM ininternal model and slope design as independent input parameters for the model, the runoff and sediment CASC2D-SED was improved.We researched the slope contribution to DEM scale effect of runoff and sediment modeling by changing the parameters ofslope alone. The small watershed, near Shagedu Town of Jungar Banner in Inner Mongolia, as the research object, we simulated theprocess of runoff and sediment using three kinds of schemes in different grid cell size taking UAV one resolution DEM data, soilcharacteristics data obtained by field survey and experiment, land use data, rainfall data, runoff and sediment data as basic inputparameter based on the improved CASC2D-SED model. The results show that (1) the simulated runoff lies between 323.18m3 and411.43m3, and fluctuation is not big within the range of 4 to 20m grid resolution; (2) the simulated erosion flow changes between3.43m3 and 65.61m3 within the range of 2 to 20m grid resolution, and fluctuation is big; (3) the slope and runoff path are two opposingfactors on DEM scale effect of hydrological process simulation and it is the main cause of DEM scale effect on hydrologicalprocess simulation without obvious changes; (4) DEM scale effect has important effects on erosion sediment, and terrain slope is themain control factor on DEM scale effect of erosion sediment; (5) the spatial variation of terrain slope changing result from DEMresolution is the main reason of the change of eroded sediment discharge in volatility.