Spectral distortions are common in quantitative terahertz spectra measurement of granulated coal. These spectra warping in the spectroscopy are caused by electromagnetic wave scattering due to coal particles and HDPE intergranularpore-space developed during tableting process consisted in the tablet. In this paper, an equivalent coal-HDPE mixture tablet is built according to the two substances’ structural differences. The scattering contributions of coal particles and air voids are investigated by using the theories of Foldy-Twersky EFA and iterative Waterman-Truell EFA in the frequency from 2 to 3.5 THz, establishing a quantitative analytical model of intrinsic absorption spectra of coal. The studied 6 samples are mixtures of HDPE powder and granulated coal, whose grain sizes varies from one another while the volume concentration is kept constant. Experimental data are recorded with a THz-TDS setup. The scattering loss caused by air voids is more notable when the size of coal particle is smaller than 38.5 μm. When the coal particle size is between 38.5 and 55 μm, it could be observed that the extinction caused by coal particles is more or less the same with it by the air voids. With the increase of coal particle diameter, the scattering contribution of granulated coal to the total propagation loss becomes evident. The correlation coefficient and RMSE between the original and calibrated spectra indicated that the studied quantitative analytical model could significantly reduce the effect of scattering to the measured THz spectrum of coal sample when coal particle size smaller than 105 μm. This study could provide a theoretical basis for quantitative terahertz spectra measurement of coal.