In order to solve the problem of low efficiency and high complexity in traditional single channel passive THz imaging system, a novel passive terahentz (THz) imaging system with single channel and optical-mechanical scanning is designed. A crank-rocker mechanism is employed to realize the fact movement of the line scanning mirror, and the two-dimensional imaging is achieved through the cooperation of the line scanning mirror and the frame scanning mirror. A Cassegrain antenna of 390 mm in diameter and 94 GHz Schottky diodes are used in this system to help to accomplish the fast scanning imaging of the objects. The single frame imaging time of the system is only 20 s, the field of view (FOV) is 30°×36°, and the angular resolution is up to 0.6°. The experimental results show that the system can image the human body, and effectively detect dangerous items concealed under the clothes. The system of fast scanning has many advantages such as low cost, high efficiency and simple structure, and provides a useful reference for the development of passive THz imager in miniaturization and high-speed.