Terahertz imaging has numerous potential application scenarios. Focal plane imaging using a fixed focal length lens and synthetic aperture imaging using an antenna are commonly used imaging methods. Focal plane imaging allows accurate focusing by adjusting the distance between the lens and the target. Synthetic aperture imaging collects data from the entire spatial field. When the transmitted and received signals are in the frequency domain, digital focusing of the echo signal is required to recover the distance information of the target. Phase calibration of the echo signal is performed using different distances to determine the distance interval of the target, and then the distance iterative step is gradually reduced to finally obtain the true distance of the target. The image sharpness determination curve can realize the automatic focusing imaging of the target. The process of iterative computation reduces the computation time by 97% compared to a direct distance search. This advantage is even more pronounced when imaging long-range targets.