This article aims to explore the flow field and sound field characteristics of the cavity structure of the dry ultrasonic cleaning head. According to the ultrasonic sounding mechanism of the dry ultrasonic cleaning head, a 3 mm cavity (small cavity) structure that generates sound through the hydrodynamic interaction between fluids and a 10 mm cavity (large cavity) structure that generates sound through the resonant interaction between fluid and acoustic mode are designed. The designed square cavity is numerically studied by the computational fluid dynamics (computational fluid dynamics, CFD) method. The results show that the two structures have similar flow field characteristics, and the flow-induced oscillation of the small cavity is more intense under the same pressure. As the pressure increases, the maximum velocity in the cavity increases and the growth rate of the maximum velocity decreases. The growth rates of the maximum velocity and the maximum velocity are similar for different cavities under the same pressure. Both structures can produce ultrasonic waves. The ultrasonic sound generation mechanism is related to the size of the cavity, and the occurrence mechanism is consistent with the predicted value. The research shows that the design of the channel structure of the dry ultrasonic cleaning head is not limited to the small cavity where the hydrodynamics interaction between fluids makes the sound, and the large cavity can also produce strong high-frequency ultrasound, which provides a reference for the structural design of the dry ultrasonic cleaning head.