Distributed models emphasize specific physical mechanism, then the input and parameters become more complex, manyproblems such as different parameters have same effect and which scale is proper, lead to uncertainty of simulated results. As forthe choice of scale, based on SWAT model and choosing Liaohe watershed as study area, this paper analysed two influencing fac -tors on estimated runoff, which include different DEM resolutions and different subbasion divisions. The results show that, 1) DEMresolution has little effect on watershed mean altitude, watershed area and subbasin number, but has much effect on mean slope,which means that when estimate runoff, we should make slope correction. As for river network, results showed when DEM resolutionis 500m, the watershed delineation is much more similar with actual river network. 2) When the quantity of subbasin divisions islittle, the estimated runoff varied considerably with the based runoff and had a sensitivity interval. This research will have muchsignificance in the development and application of distributed models.