The structure of three-dimensional (3D) preforms is the key to the performance of 3D reinforced composites. In order to improve the quality and efficiency of manufacturing, this paper originally proposes the ultrasonic vibration-assisted multi-cycle compaction method. Ultrasonic vibrations are applied, using a resonant 40 kHz compactor, to the compaction of 3D carbon fiber preform. Compared to the traditional method, the ultrasonic vibration-assisted multi-cycle compaction method can accelerate stress relaxation and reduce preform springback. The microstructure of preform is observed using x-ray computer tomography imaging. It elucidates the mechanism by which ultrasonic vibration promotes fiber slippage. The compaction forming experiment of preforms has proven that the ultrasonic vibration-assisted multi-cycle compaction method can reduce the compaction time, improving the forming quality. This can improve the technical support for the improvement of the manufacturing level of the 3D preform.