In the terahertz and far infrared region, aluminum is in a state of transition from conductor to dielectric, and the research of the interaction between aluminous target and electromagnetic wave is meaningful for scattering prediction of targets. With the available error criterion model, dielectric function of aluminum is determined by fitting to experimental data in the terahertz and far infrared region. The transmitted parameters in aluminum are deduced by considering different loss mechanisms. Reflection and transmission characteristics on the interface of aluminum are investigated, and the reflection coefficients are given as a function of frequency. Results show that the transmitted parameters in aluminum keep their continuity and coherency from microwave to terahertz frequency. RCS (radar cross-section) results of aluminum plates computed by IBC method demonstrate that the increased wave impedance of aluminous targets has little impact on its backscattering, and the polished aluminous plate or sphere can still be treated as a perfect electrical conductor and used as a reference for RCS calibration.