Two important issues must be considered when modeling the terahertz (THz) wave scattering behavior using microwave EM methods. The first is the material response characteristics, including the metallic materials that may be beyond the scope of the Drude model’s description and the dielectric materials that may lack appropriate description models. The second is the modeling method for investigating the THz scattering behavior of the surface random roughness and the complex fine structures. Several theoretical endeavors are presented in this paper to elucidate the surface- and volume-scattering phenomenon observed in the experiment data. First, we employ the Integral Equation Method (IEM) to fit the measured data of an aluminum plate. Good agreements confirm the superior applicability of IEM to the metallic materials. Nevertheless, for dielectric materials, the volume-scattering contributions of the inner microstructures or particles whose sizes are comparable to the THz wavelength are required to be considered. Furthermore, our findings state that, for dielectric materials, it can be fit to the experimental data with the help of the Vector Radiative Transfer (VRT) theory. Finally, this research proposes a semi-deterministic description-based ray-tracing high-frequency algorithm to realize the rapid modeling of the coherent and incoherent scattering of electrically large complex targets at THz bands.