The objective of this paper is to explore the relationship between the characteristics of plastic particles in suspension and acoustic impedance spectrum and to present a novel non-invasive methodology for both spherical and non-spherical particle sizing. By modifying the ultrasonic attenuation spectral model, theories relating acoustic impedance spectrum to particle characteristics have been established to implement quite a few numerical simulations for the first time, revealing that the acoustic impedance of plastic particles is sensitive to changes in particle concentration and size. Afterwards, experiments were carried out on polystyrene suspensions made by particles with different sizes. On the basis of the theoretical analysis, different transducers were employed over a frequency varied from 10 MHz to 100 MHz for different particle sizes respectively. Not only were spherical particles chosen for the experiment, but also non-spherical particles with three different size distributions considering the fact that practical particles have irregular shapes. All the samples were verified by optical microscope technique and their comparisons with the experimental results show that the plastic particles with different sizes are distinguishable by using acoustic impedance spectrum.