Polymer vane extruder is a new type of extrusion machine, whose plasticating and conveying method is based on elongational rheology. The vane extruder has many advantages over conventional plasticating method, such as shortening thermo-mechanical experience of plasticating and conveying, reducing the energy consumption of plasticating and conveying, and improving adaptability to diverse materials. Currently, research for characteristics of vane extruder is mostly focused on theoretical derivation and experiments. However, these methods lead to high consumption of time and energy, and cannot provide an intuitive view of the flow. In this paper, 3D finite element numerical simulation was performed to investigate the flow in one of the vane units with POLYFLOW. The elongation flow was investigated by analyzing velocity field, pressure field, local-shear-rate distribution and mixing index distribution. The results show that numerical simulation is an efficient method to optimize process parameters and the structure of vane extruder.