An active imaging system has been constructed by using a Cassegrain reflector with the diameter of 30 cm. The system works at room temperature and 0.22 THz. The detectors are self-made Nb5N6 microbolometer THz detector and commercial VDI detector. Images for 5 parallel copper rods with the diameter of 1.4 cm and space of 5 cm are got and analyzed. The imaging time is reduced from 15 s to 7.5 s by changing the reference frequency and scanning velocity. The range of the image is 20 cm×6.5 cm. The special resolution of the images is about 1.41 cm for the two detectors.