A stepped frequency imaging radar which based on 0.22 THz high-power gyrotron oscillator was designed. The dual Cassegrain antenna with separate transceiver was used for transmitting stepped frequency pulse signal. Frequency adjustment of gyrotron oscillator was achieved by adjusting gyrotron electron beam voltage or superconducting magnet strength. The design schemes for imaging radar were given， the radar detected range and high-resolution range profile of the stepped frequency radar system were analyzed， and inverse synthetic aperture radar （ISAR） imaging simulation was carried out. The results of theoretical calculation and simulation show that the radar could detect the target with scattering area of 0.01 m² by 1.982 km and distinguish the targets with a distance of 2 cm at 500 m. The radar can be used for investigations on target recognition of small objects such as UAV which can take concealed weapons and threat material.