3D geological modeling is the key to realizing BIM dynamic design of geotechnical survey,and how to establish a 3D geological model that can be applied directly to guide design and construction is a one of the difficult points in the engineering practice.Taking Xiamen second east-tunnel project geological investigation for example,based on the field surveying topographic data and the geological data of drilling and geophysical prospecting,etc.,and using the surface feature of Civil 3D,we create the terrain surface and formation surface at first,and then pick entities through the surfaces to create the final 3D geological model layer by layer.In addition,through the secondary development technology,we establish the arbitrary cut-off supplementary module based on Civil 3D to realize arbitrary sectioning of the geological model and generation of the grid diagram.Moreover,the trench excavation design of the immersed tube tunnel is performed through the component editor.The engineering application results show that the modeling method is convenient,intuitive and efficient,and the resulting model can be directly applied to guide engineering design and construction.