Abstract: As one of the most important variables of topography,the quality of extracting flow length has a serious effect on soil and water conservation,soil erosion and environmental assessment studies.The error of extracting flow length from grid-based Digital Elevation Model is mainly from original DEM and extracting algorithm.Based on the principle of data independent DEM interpretation analysis method,this paper quantitatively studies the error spatial distribution of flow length which is extracted from grid-based Digital Elevation Model.The result shows that the direct calculation algorithm,which is widely used,has a better effect on the calculation of the catchment area than the valley area,and at the water outlet the error achieves its maximum;flow length of the valley area contains some system error.