Abstract: FLAC3D is a widely used software in geological numerical simulation. There are several basic steps to realize the three-dimensional modeling study, establishing the model, defining the constitutive relationship and parameters, setting the initial and boundary conditions, and finally solving the stress field. However, as the complexity of the geological conditions, building up an accurate and complex 3D geological body is always a difficult problem in pre-processing of FLAC3D.Therefore, a new model-building-up method is put forward in this paper, to set up grids with the powerful matrix functions of MATLAB and the powerful interpolation functions of SURFER.The result showed a high matching between the model and the real topography and geological condition. This new method is simple, practical and easy to operate, which makes it a good approach in pre-processing of FLAC3D.