Abstract: 3D woven composites are widely used in the aerospace field. As a reinforcement structure, the geometry of fiber preform has a decisive influence on the mechanical properties of composites. However, a preform is a flexible structure that is prone to significant geometric variation during the molding process, including yarn path changes and compressive deformations of cross-sections. Achieving refined and high-fidelity modeling of preforms is an important prerequisite for performance prediction and structural design of composite materials. Aiming at modeling of the complex fiber structure for carbon fiber 3D woven preforms, a quasi-fiber scale modeling method based on the concept of virtual fiber was proposed. Movements and deformations of yarn in the weaving process were simulated, and high precision model of 3D woven preform was constructed. The Micro-CT technology was used to analyze the unit cell structure inside the preform sample, which verified the reliability of the model.