In this paper, a stepped-frequency continuous wave synthetic aperture radar imaging system based on millimeter wave is studied. Based on the algorithm, the imaging hardware and software system are built to realize the signal scanning and the acquisition and processing of echo signal. The processed signal is calculated by the imaging algorithm, and the 3D image data of the target is obtained. The results show that the resolution in x direction is 8~9 mm, the resolution in y direction is 12 mm, and the resolution in z direction is about 40 mm. The optimal imaging distance of the system is studied, and the results show that when the target is within the range of 5~16 cm from the transceiver, the target can be clearly imaged with only a small background noise. In the application of human security inspection, the penetration effect of five common fabrics is tested. The results show that the system can effectively penetrate the fabric and clearly image the hidden objects under normal clothing thickness.