Terahertz waves have a wide range of applications in biology and hazardous explosive detection, but they suffer limitations in imaging owing to the sensitivity of the corresponding detectors. Herein, we demonstrate a self-mixing coherent imaging system with a terahertz quantum cascade laser. We achieve imaging with a large depth of field in the terahertz band by combining the holographic imaging technology and self-mixing interferometric measurement. We imaged an area of 2 pair line per millimeter on a resolution board in the off-focus plane in two dimensions using a feature like self-mixing of a transceiver. We obtain the same image contrast as that on the focal plane when the imaging plane is 56 times the wavelength offset.