Extracting the to pographic moisture index from medium and low resolution DEM has an important significance to the research o n regional soil erosion factors and mo dels. How ever, as the resolution becomes lower, slope tends to become gentle and unit grid elevation information changes will also affect the calculation of unit catchment area.So,extracting the to pographic wetness index at medium-low resolution must take into account the impact of these two aspects.The 10,20,40,and 60m resolution DEM s are separately interpolated from digital topographic map on 1:50 000 scale and then statistics frequency and cumulative frequency are analyzed. Slope transformation is made based on the 10 m resolution DEM . T he average of catchments area of hig h-resolution DEM raster is taken as the unit catchment area of the medium-low resolution DEM. Finally, the topographic moisture index is corrected according to the above mentioned two aspects.