A Vector Network Analyzer (VNA) is an indispensable piece of equipment for the electromagnetic characterization of materials. With continuous development, the testing frequency band of the VNA has entered the terahertz domain. Previous studies have shown that carbon nanofibers (CNFs) prepared by the vapor method have excellent absorbing characteristics for both the microwave and the millimeter wave. This study is the first to characterize CNFs using a VNA in the 500–750 GHz band. The results show that CNFs with a random orientation distribution have an obvious polarization-sensitive absorption behavior in the 500–750 GHz band test, and this effect also causes a phenomenon of polarized reflection. As a comparison, the CNF sample was also tested in the W-band by the same test method. However, the result tested in the W-band showed a strong absorbing characteristic without any polarization response. The mechanism of the above phenomena was analyzed from the perspective of crystal growth and the principle of the antenna and polarizer. For further exploration and proof, CST Microwave Studio was used for the first time to model and simulate CNFs according to the actual parameters of the CNF sample.