Abstract: On the basis of image classification, the computer algor ithms were developed to remove the image isolated islands, extract the image edg e, thin the wide edge,and analyses image area and image shape with the help of c omputer automatically.Finally the real edge pixels were obtained.All these proce sses laid the foundation for producing the thematic map of forestry.The comparis on and analysis were made for the subcompartment edges based on auto-produced i mage and the map obtained from field survey,the feasibility of computer algorith m can be accepted.