Active millimeter wave imaging provides a safe and effective technique for human security inspection. In order to further improve the imaging resolution and quality, the development of millimeter wave imaging system with wide band and high frequency is of great significance. For this purpose, an active submillimeter-wave imaging system with wide band and high frequency which utilizes a 280 GHz– 320 GHz transceiver is constructed and characterized. It uses the heterodyne mixing technique to directly get the holographic data which can be reconstructed to form the reflectivity image of object. The range and cross-range resolutions are tested as well as the signal to noise ratio of the system. The range and cross- range resolution reaches 4 mm and 2.5 mm×2.5 mm, respectively, approaching their theoretical values. The impact of bandwidth on the imaging results is also investigated benefiting from the wide-band nature of the system. Apart from increasing the range resolution, wide-band nature can also effectively suppress the speckle effect.