Distributed hydrologic models use spatially variable input parameters to predict watershed response, and a widely used method for representing watershed is to aggregate input data on the basis of subwatershed. However, model output could be affected by the spatial extent over which input data are aggregated to produce parameters. It was hypothesized that model output stabilized at the point where spatial characteristics of watershed were fully considered. With the support of GIS technology, distributed hydrologic model--SWAT (Soil and Water Assessment Tool) was used in two watersheds with different area to model the impact of different subdivisions of watershed on runoff and sediment yield, and to some extent the results validate the above hypothesis. Then reasonable subdivision level of watershed for distributed hydrologic model was discussed, increasing the subwatershed number beyond this level does not significantly affect the runoff volume and sediment yield. This reasonable subdivision level could be used to optimize input data preparation requirements and simplify the interpretation of results without compromising simulation accuracy.