In order to improve the accuracy of ultrasonic application in hydrogel environment, the influence of sodium polyacrylate xerogel particles in the swelling process and of the different packing state on ultrasonic field was studied by using a self-built ultrasonic field experimental platform. Besides, the effect of hydrogel particle of different state under ultrasonic field on sound intensity attenuation was also investigated. The results show that when the xerogel particles swelled by water for 150 s, the sound intensity is almost stable, with the sound attenuation amplitude reaching 55.00%. The influence of swelling of xerogel particles in different packing states on sound intensity attenuation is consistent: the sound attenuation amplitude increases first and then reaches a plateau. And the larger the packing density of particles is, the smaller the sound attenuation amplitude is at the stable state. Under ultrasonic field, as the hydrogel particles gradually saturated with water, the resulting amplitude of sound attenuation gradually decreases and finally stabilizes at 7.97%.