In order to rapidly optimize the ultrasonic-assisted underwater nanosecond laser cutting process, the impact of cavitation bubble dynamics characteristics (CBDC) on the process outcomes was analyzed. And theoretical analysis and experimental validation were carried out using numerical simulation analysis, orthogonal experiments, and high-speed imaging methods. Optimal parameters for the ultrasonic-assisted underwater laser cutting process were obtained, and the CBDC was confirmed to be the primary factor affecting the cutting process. The results showe that as the interference of cavitation bubbles with the laser beam increases, the cutting depth decreases while the cutting speed increases. As the depth-to-width ratio of the groove increases, the pulsating shock from cavitation bubbles exerte greater equivalent stress on the groove bottom. The maximum depth-to-width ratio of approximately is 1.71 achieved when ultrasonic power P_u=65 W, water layer thickness h_w=1 mm, laser pulse frequency f_q=20 kHz, and laser scanning speed v=1 mm/s, respectively. Under these conditions, the groove width is approximately 99.88 μm, the groove depth is approximately 170.18 μm, the size of the heat-affected zone is approximately 31.71 μm, and the microcrack length is approximately 33.42 μm. At this time, the cavitation bubble cycle is shorter (approximately 100 μs~160 μs). This research can provide valuable insights for optimizing the parameters of multi-field underwater laser composite processing.