In order to study the influence of interface reflection on terahertz (THz) spectroscopy of double compression tablets, the THz time-domain spectroscopy (THz-TDS) was used to detect the single- and two-layer sand tablets made of different particle sizes, and the THz-TDS were obtained. The results show that the THz peak attenuation coefficient of the unit thickness of the double-layer sample was significantly smaller than that of the single-layer sample. Therefore, a coefficient should be added to study THz spectral response of double-layer samples to eliminate the loss caused by interface reflection of samples, and the coefficient was related to the characteristics of materials.