Abstract: THz laser active imaging is a kind of new imaging system. It has great potentials in hazardous material detection, medical care and food inspection because of its low photon energy (about 4.1meV per THz) and high penetration capability. In this paper, the contrast stretching transformation, median filtering and mean filtering are applied to pre-process the THz array image of a 5RMB watermark. The processing results show that these imaging processing methods could be introduced to the preprocessing of THz laser active image and satisfactory results are obtained.