To pursue high-performance computer numerical control (CNC) milling of complex parts, it is crucial to simulate their machining process geometrically and physically with high fidelity beforehand. The geometric simulation is to construct three-dimensional (3D) models of the finished parts and to compute geometric deviation between the models and the part designs, in order to verify the planned tool paths. The physical simulation is to build undeformed chips geometric models and in-process workpiece models and to compute instantaneous cutting forces, in order to optimize the machining parameters. Therefore, it is essential to accurately and efficiently model undeformed chips geometry in machining complex geometric parts. Unfortunately, this work is quite challenging, and no well-established method for this work is available. To address this problem, our work proposes an accurate and effective approach to 3D geometric modeling of undeformed chips geometry in three-axis milling of complex parts. The outstanding feature of this approach is that undeformed chip models and in-process workpiece models can be effectively constructed. This approach lays a theoretical foundation for the geometric and the physical simulations of three-axis milling. It advances the technique of machining simulation and promotes high-performance machining of complex parts.