Abstract: As an important approach of the terrain analysis in GIS,digital elevation model(DEM) is prime and unique.In order to analyze the terrain conditions of the subcompartment for planning and designing in afforestation scheme,the regular digital grid model and regular digital plot model about elevation were established from contour line.And based on it,terrain factors,needed for site condition in afforestation decision support system(ADSS),were automatically calculated by computer.As an example,the calculational methods of slope aspect of subcompartment were presented in the developing environment of (MapObjects).After analyzing results through such methods,the automatic account of terrain information based on statistical theory is best.It is also the best solution in terrain analysis of spatial decision support system(SDSS) based on COM GIS.