We present a new approach for constructing the initial 3D geological models in the process of man-machine interactive interpretation for gravity and magnetic anomalies. Firstly, we introduced the steps of method. It includes some auxiliary vertexes and sections techniques. Then, the forward algorithm of the model is given. And the data structure of the model is devised, later the modifying method and visualization method is discussed. This method is realized in our 3D gravity & magnetic anomaly interpretation system based on VC++6.0 and OpenGL. Using this method, the geophysical interpreter can construct or modify the geological models easily under the three dimension environment. The tool can give them visual 3D models, so it enhances the efficiency of the interpretation.