In this study, fully-polarimetric information is introduced to a standoff holographic imaging system. A 0.14-GHz fully-polarimetric wideband radar is designed for detecting concealed targets in a standoff fully-polarimetric holographic-radar-imaging experiment. The low-entropy polarization scattering of concealed targets is analyzed through the target-polarization decomposition method, and the average polarization scattering mechanism is investigated at different irradiation angles. The experimental results show that in the 0.14 THz band, the covering by the typical clothing materials is found to alter the speckle pattern of the background clutter, but has a negligible impact on the polarization scattering of a strongly scattered echo target. The polarization scattering entropy of the artificial target is smaller than that of the background clutter. The main scattering mechanism of a model pistol is the low-entropy surface scattering from its smooth surface, in which the inclined stripes on the sleeve contribute to the low-entropy multiple scattering, which is the main source of the cross-polarization echo of the target.