Terahertz time-domain spectroscopy is used to research the intermolecular or intramolecular interactions and some optical properties, such as refractive index, dielectric constant and absorption coefficient. As the dopant in terahertz band, non-absorbing particles, such as polyethylene or others, are usually mixed with pure biological samples by compressing tablets. Due to inhomogeneity and different particle sizes in the tablets, the unobvious absorption from pure sample was affected by doped particle in mixtures. In order to extract the permittivity of pure sample from mixture, Bruggeman effective medium approximation (EMA) theory can be applied. The optical constants and the permittivity of the pure sample can be obtained by using EMA from a composite medium of biological sample and polyethylene. EMA is employed in this work and the relationships between the calculation results and particle sizes are to be explored. It shows that the practicability of Bruggeman effective medium theory in the identification of terahertz spectrum of mixture.