The translucency of terahertz wave through many polymer composite materials, particularly foams, makes it a good complement to another nondestructive testing (NDT) methods. Terahertz time-domain spectroscopy (THz-TDS) technology can obtain amplitude and phase information simultaneously. It is a powerful tool for nondestructive testing of foam materials to embody the depth information of defects while acquiring defect location, size, and nature. In the process of NDT, the refractive index is one of the important factors that affect the detection effect. In previous studies, the effective refractive index of polymethacryl imide (PMI), a commonly employed foam material, in the THz band is found to be very close to that of air. However, based on systematic measurements of the THz spectra and scattering parameters of PMI with a variety of thicknesses and densities, it is found that THz attenuation in PMI has a strong frequency dependence and that the size of the particles making up the material has a great influence on the attenuation: the larger the size of the micro particles and the faster the attenuation of the THz wave. This paper attempts to explain this phenomenon through combined experimental study and theoretical analysis, supported by the numerical simulation based on the Mie scattering model.