The THz reflection spectra of optically thin hexogen (RDX) samples were studied by terahertz imaging with spectral resolution. A photoconductive antenna excited by femtosecond laser radiation was used as a source of broadband THz radiation. The presence of a Fourier spectrometer (as well as band-pass THz filters) and a microbolometric THz video camera in the experimental setup made it possible to use the terahertz imaging method to study reflection spectra taking into account scattering in the range of 0.6 to 1 THz. The influence of the optical characteristics of the substrate on the THz reflection spectra was studied. In particular, the conditions for observing the effect of anomalous dispersion for RDX samples with different dispersion in the frequency region of the RDX absorption band 0.8 THz were studied. The obtained results demonstrate the application of the method of terahertz imaging with spectral resolution based on THz video camera for the identification of explosives with concentrations 0.75 to 50 mg / cm² on various surfaces.