Abstract: In view of some relief symbols are difficult to build the modeling in 3D topographic map,we take cliff symbol as the example to propose a method to exact the edge lines of cliff based on TIN,set up an illumination model of cliff symbol according to the visual sensation rules under the different illumination conditions and research the mathematical expression of it,and introduce the automatic modeling flows of cliff symbol.The experiment proves that it is feasible.The study will be helpful to build the modeling of other similar relief symbols automatically.