Plasma sheath formed during the reentry of near space hypersonic vehicle will interfere the electromagnetic wave detection. In order to solve this problem, the model of the typical near space vehicle was built, and the flow field distribution under different flight conditions during the reentry process was simulated. Based on the flow field distribution, the plasma parameter distribution was modeled, and the transmission characteristics of terahertz wave in the plasma sheath were theoretically calculated by using the scattering matrix method. The results show that high frequency terahertz wave can penetrate plasma sheath effectively. In addition, terahertz active imaging experiments were carried out in the laboratory environment. The experimental results show that the plasma exerts little effect on the terahertz active imaging results. The simulation and experimental results preliminarily prove the potential of terahertz technology for hypersonic vehicle detection, which is of great significance for national defense.