Abstract: In recent years, active near-field terahertz imaging has been intensively investigated for personnel surveillance. There are mainly three imaging modes, planar scanning, cylinder scanning and fan-beam scanning. In this paper, the imaging principles of the three modes are introduced and advantages and disadvantages of each mode are also discussed. Moreover, we give an overview of the factors that affecting the image quality. The approaches to achieving better images are analyzed from the aspects of resolution, signal to noise ratio and artifacts removal.