Ultrasonic cavitation has garnered extensive attention for its potential in surface modification due to its characteristics such as microjet impingement and high-pressure shockwaves generated during bubble collapse. Although the collapse dynamics of a single bubble have been widely examined, researches on the global fluid behavior remains insufficient. This study establishes a numerical model of ultrasonic cavitation with the aim of systematically examining how ultrasonic amplitude, ultrasonic frequency and standoff distance affect the bubble distribution and the absolute pressure in the fluid domain. On this basis, experiments were also conducted using a self-developed ultrasonic vibration device to investigate the cavitation erosion on aluminum alloy by measuring the surface morphology and roughness. The results show that the cavitation bubbles are primarily distributed within 0.7 mm beneath the tool end, reaching a peak absolute pressure of 4.22 MPa when the standoff distance is 1 mm. When the ultrasonic amplitude is increased, the peak absolute pressure is enlarged from 0.74 MPa to 3.53 MPa, the peak vapor volume fraction is increased from 6.5 % to 34.8 %, and the surface roughness of eroded workpiece is raised from 0.110 μm to 1.013 μm, with surface morphology evolving from microscopic pits to large-scale material removal. The vapor volume fraction reaches the maximum value of 97 % at a standoff distance of 0.25 mm, but sharply decreases to 0.08 % at 2 mm. The workpiece is most severely eroded to the largest roughness of 1.013 μm at the standoff distance of 0.5 mm. When the ultrasonic frequency is brought up, the peak absolute pressure is progressively increased, but the peak vapor volume fraction is diminished with accelerated change rate. Low-frequency ultrasonic vibration produces stronger impacts by fewer large bubbles, whereas high-frequency ultrasonic vibration generates weaker impacts by numerous small bubbles. The simulation and experimental results demonstrate strong agreement with each other, which provides deeper insights into the erosion mechanisms induced by ultrasonic cavitation. This article will enrich the fundamental theory of ultrasonic cavitation and provides theoretical support and guidance for material modification using ultrasonic cavitation.