Abstract: In order to understand the dynamic characteristics of cavitation bubbles in a Helmholtz nozzle cavity and the evolution of cavitation bubble responses under the influence of ultrasonic waves, a mathematical model describing the dynamic variation of cavitation bubbles in a self-excited oscillating nozzle cavity was developed based on cavitation dynamics. In addition, the effects of Helmholtz nozzle cavity length and diameter on cavitation intensity and the dynamic behavior of cavitation bubbles when subjected to an additional acoustic field were studied. The results showed that both the cavity length and cavity diameter of the self-oscillating jet nozzle affected the cavitation intensity in the cavity. The increase in the cavity length and cavity diameter contributed to improving cavitation intensity. The expansion and contraction of cavitation bubbles in the acoustic–fluid coupling field were more severe than those in a single flow field. The frequency and amplitude of ultrasonic waves also had a great influence on cavitation intensity, with an optimal ultrasonic wave frequency identified for maximizing cavitation intensity in the cavity. In addition, excessively high ultrasound frequencies resulted in shorter acoustic wave expansion time and a shorter growth time of the cavitation nucleus. There was a positive association between cavitation intensity and acoustic field amplitude. These research findings are valuable for enhancing the practical application of self-excited oscillating cavitation jet technology and ultrasonic-enhanced pulse jet technology.