Active terahertz-wave imaging systems play a significant role in security and surveillance applications. A 0.14 THz near-field imaging system is described, which consists of a signal generator and acquisition unit, a transceiver front end, a digital signal process unit and a motor control unit. Based on the two-dimensional synthetic aperture technique and the image reconstruction algorithm, this system is capable of producing three-dimensional images of 2 mm lateral resolution and 3 cm range resolution in concealed weapon detection experiments.