With an aim to enhance the ability of the abrasive waterjet machining, the ultrasonic assisted micro abrasive waterjet machining system was designed. The pre-mixed device was used as the abrasive supply device. The ultrasonic nozzle device is the key to the system. The processing mechanism was studied. Pulsed waterjet with cavitation effect was generated with the ultrasonic vibration. The ability of the abrasive waterjet was enhanced with the effect of high dynamic pressure of pulsed jet and cavitation. The horn is the core component of ultrasonic device. The performance of the horn was verified by finite element analysis to ensure that the horn works effectively and accurately. The erosion experiments prove that the effect of ultrasonic vibration can improve the impact force and the material removal rate. However, with the increase of the amplitude, the surface quality decreases.