The influences of materials properties and surface roughness on the scattering computation of targets are considered at terahertz frequencies. For the geometric modeling of rough targets, the conventional fine facets splitting strategy will generate a large number of facets, making the solving of scattering calculation unaffordable. An efficient method combining deterministic modeling and statistical modeling is proposed. Then method based on Full-Wave Approach(FWA) for the scattering computation of arbitrary rough convex targets is realized at terahertz frequencies. The proposed scattering prediction method is verified by the simulations of standard targets comparing with the results of commercial electromagnetic computational software(FEKO).