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Terahertz reflectance spectrum is widely used in material thickness measurement, material composition detection and other aspects. However, the reflection wave is easily affected by the surface roughness produced in the process of material processing, resulting in noise and measurement error. In this paper, the terahertz reflection model of three-layer medium is derived, and the relationship between smooth surface and rough surface is established by Kirchhoff approximation theory. Secondly, the terahertz reflection spectrum and frequency spectrum of aloe emodin were measured by THz-TDS(terahertz-time domain spectral system). By introducing controllable roughness surface with sandpaper of different specifications, the reflection spectrum of roughness of each specification surface was measured, the reflectance corresponding to different frequencies was calculated, and the reflection spectrum of smooth surface was compared and analyzed. The influence of these results on the ability of THz reflection spectrum to detect Chinese medicinal herbs was discussed. In addition, by introducing Kirchhoff approximation, the reflection spectrum relation model of rough surface and smooth surface is established to compensate the reflection spectrum of rough surface. It is found that the power of reflection spectrum can be improved to some extent and the influence of rough surface on reflection spectrum can be reduced.
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