Three-dimensional (3D) geological modeling based on borehole data usually requires many manual operations, and the modeling process remains very complicated and time consuming. This paper presents an automatic implicit 3D geological modeling and visualization method that can be applied to urban geotechnical drilling data. First, by analyzing the general characteristics of these types of data, the implicit Hermite radial basis function (HRBF) surface is used to simulate geological interfaces according to lithological layers/units in boreholes. Second, the marching tetrahedra (MT) visualization method is optimized with new data structures and split operations to extract explicit mesh models from implicit geological surfaces. Then, the optimized tetrahedron used in the field of 3D urban drilling modeling is described in detail. Finally, the local geotechnical engineering data of two Chinese cities are used to verify the modeling algorithm, and several kinds of spatial analyses are carried out, including visualization, data querying, accuracy assessment and multiple cutting methods. The results show that the proposed urban subsurface 3D geological modeling method is effective in visualizing the structural shape, topological relationship and formation properties of strata that can be used to predict underground conditions and reduce construction risks during urban geotechnical engineering projects.