<p>Longitudinal ultrasonic vibration of the electrode significantly improves the aspect ratio of micro-holes and reduce the occurrence of harmful pulses, such as arcing, in ultrasonic vibration-assisted Electrical Discharge Machining (USV-assisted EDM). Most studies have demonstrated that pronounced ultrasonic cavitation at the electrode tip and the enhanced pumping effect facilitate the rapid evacuation of machining debris, thereby improving machining efficiency. However, once the vibration amplitude exceeds a critical threshold, further increases in amplitude lead to a gradual decline in the material removal rate (MRR). We propose a new hypothesis suggesting that the reduction in MRR is primarily due to the decrease in effective discharge energy per pulse. This study elucidates the effective discharge energy mechanism in ultrasonic vibration - assisted EDM by analyzing the morphology of generated crater and arc plasma duration at varying discharge time via in situ high-speed camera imaging and computational fluid dynamics simulations. Our findings indicate that at large vibration amplitudes, the discharge waveform is divided into multiple segments, each separated by short-circuit waveforms. Among these, only the initial segment of the waveform corresponds to discharge energy that contributes to material erosion, while the subsequent arc plasma extinguishes rapidly, ceasing to affect material removal. Compared with the beneficial effects of ultrasonic cavitation and the pumping effect on machining performance, the reduction in effective discharge energy per pulse at large vibration amplitudes becomes the dominant factor responsible for the decrease in MRR.</p>