A method to overcome the problem of limited dynamic range in current radar-cross-section (RCS) measuring systems in the terahertz band was proposed, by adopting structural subsection calibration and data subsection processing. The limitation of onefold calibration on the RCS measurement range and accuracy was analyzed based on the relative calibration theory, and verified by experimental measurement of a smooth metal sphere and a smooth metal cylinder. A smooth metal plate was also used to discuss the effect of using signal attenuators on the system measuring range and accuracy. Finally, the measurement of the RCS of a warhead model was realized by structural subsection calibration combined with data subsection processing, and the main scattering characteristics were successfully recognized. The measuring dynamic range reached 63 dB, which should be the highest as far as we know. The measured results accorded well with the simulation results.