Studied the effect on micronization of cellulose in 20 kHz ultrasonic crushing sound field conditions. It was determined and selected the geometric parameters of the ultrasonic resonant reactor for cavitation crushing field by using the analysis of sound propagation in a cylindrical pipe. Combining with the acoustic modal analysis, the height of liquid level foe resonant reactor was obtained, which could enhanced acoustic energy density distribution. Used the finite element method to analyze the acoustic field of ultrasonic material crushing cavitation. Finally, The ultrasonic crushing experiments were conducted with the resonant reactor liquid level and the non-resonant reactor liquid level by different power conditions. Results, the acoustic energy density distribution in the sound field of the crushing cavitation, under the condition of resonant reactor liquid level, was closely related to the acoustic mode. The acoustic energy density was significantly enhanced, the distribution of cavitation was expanded, and the cavitation crush effect was obviously enhanced. The results of cellulose sonication experiments showed that, under the condition of low power (＜140 W) and short processing time (＜7 min), the average size of D_\[3,2\] and D_\[4,3\] of cellulose treated by ultrasound in the resonant reactor liquid level was significantly reduced. However, with the increase of ultrasonic time and ultrasonic power, the acoustic flow would enhanced inside the cavity, and cavitation effect caused by the ultrasonic field almost no longer follows the linear sound field characteristics, the crush energy distribution in the resonant structure was disturbed by the nonlinear acoustic flow, which was similar to the energy distribution characteristics of the non-resonant ultrasonic field. As for the particle size difference of materials under the same treatment condition, although it was significantly narrowed, the resonant sound field still had a good crushing effect.