In this paper, a passive quasi-optical terahertz imaging system for a human body based on a single detector with fast scan mechanism is presented. A resolution test chart for image quality evaluation is designed. The proposed imaging system comprises several reflection quasi-optical devices. Terahertz waves emitted from the human body were transmitted through a terahertz window into the imager and were sequentially reflected by a flapping mirror, rotating mirror, focusing mirror, and fixed mirror and finally reached the terahertz detector. The imaging system can obtain a full image of 0.65 m×1.9 m using only one detector, with a resolution of 2 cm and an imaging distance of 1.5 m within 2.5 s using a fast scanning mechanism comprising a flapping mirror and a rotating mirror. The imaging system is capable of displaying the profiles of suspected dangerous items carried on the human body, such as metals, liquids, ceramics, and powders. The detected items are automatically marked by software.