Terahertz wave (THz), which exists between microwave and far infrared, has made the terahertz spectroscopy and imaging technology develop rapidly in recent years because of its non-damage property and high stability. The unique feature of non-invasive detection of terahertz wave has a perfect prospect in the field of security detection, and has attracted the attention of scholars. Although THz images obtained by terahertz imaging system can identify hidden weapons or other metal products, the THz image contrast and clarity are poor, which can not completely accord with human visual effect, and is not conducive to machine recognition. At present, the enhancement and improvement of terahertz image quality are the key to the long-term development and wide application of THz imaging technology. By using terahertz projection imaging system with scanning step 0.5mm, we imagine the objects of the metal pendant and metal arrow hidden in clothes at the same experimental conditions. Because of the system defects such as terahertz light source, energy fluctuation as well as the complexity and interference of the external environment, the imaging of the imaging system we get has serious background noise and blurred boundary, and the imaging quality is poor. This paper presents an algorithm of Canny equalization based on double threshold for THz image enhancement. According to the property limitations of terahertz image itself, this algorithm determines the relevant thresholds and the range of image equalization to realize the image denoising. The algorithm introduces the double threshold Canny algorithm and the gradient amplitude algorithm to improve the contrast and clarity of the image, preserve and highlight the detail information of the terahertz image, as well as obtain the high resolution and clear edge image. The experimental results show that we successfully obtain the terahertz images with improved definition, lower noise and fully detailed information from tow low-resolution (LR) terahertz images. The quality of images is improved.This work also enhances the discrimination ability and perspective capability for the hidden defects or hidden objects in the terahertz images, which demonstrates the enormous potential that the terahertz imaging provides the application in the security check.