Deep rock masses exist in a complex environment with multi−field coupling; therefore, it is necessary to develop a true−triaxial static−dynamic−coupling loading test machine to explore their characteristics and mechanical response mechanism. To meet the test requirements of true−triaxial loading and strong disturbance, a wave−absorbing metal plate was selected as the boundary material between the granite and transmission end, and the modified SHPB was used to perform static−dynamic−coupling loading tests. In this study, two series of experiments on wave− absorbing metal plates were conducted, which were fixed aperture sizes with different thicknesses and fixed thicknesses with different aperture sizes. The static−dynamic−coupling loading tests on each aperture size and plate thickness were carried out under the condition of equal energy impact. The effects of the aperture size and plate thickness on the incident− and reflection−stress curves, reflectivity, energy consumption law, energy evolution, and other mechanical properties of the wave−absorbing metal plate materials were studied. The results show that the peak stress and reflectivity decrease with increasing aperture size and plate thickness, and the influence of the thickness is greater than that of the aperture size. The energy−absorption rate of the wave−absorbing metal plate increased with increasing thickness and aperture size and was maximized when the aperture size and thickness were 6–7 mm and 3–4 mm, respectively. The variation trend of the energy reflectance is opposite to that of the energy absorption and reaches a minimum when the aperture size is 6–7 mm and plate thickness is 3–4 mm. The energy transmittance of the wave−absorbing metal plate fluctuated in a stable range, but the variation range was less obvious compared to that of the energy−absorption rate.