Abstract: The THz image enhancement, segmentation and contour extraction algorithms are studied to achieve effective target image detection. Firstly, the terahertz images are enhanced and their grayscales are stretched. Then we apply global threshold segmentation to extracting the target, and finally the targets are marked on the visible image. The experimental results show that the algorithm proposed in this paper can extract and mark targets effectively, so that people can find suspicious objects under clothing more quickly. The algorithm can improve the usefulness of the terahertz security apparatus we have developed.