According to the characteristics of THz images, some digital image processing methods such as threshold division and edge detection are used to segment the pistol from a THz image. Then, the effective signatures of the pistol are extracted and automatic recognition and judgment are implemented on the image. If hazardous or suspicious goods are found, an alarm signal is generated. The experimental result shows that this method can effectively discriminate the pistols concealed in clothes from other metal goods such as pens, keys and ornaments which people usually carry. It can be used to automatically identify the contraband goods such as pistols in the THz images in a security check.