Abstract: The present application discloses a measurement unit for holding a sample (including a powder sample in small quantity, a scattered powder sample or an aqueous solution sample) for a terahertz beam irradiation; and a terahertz spectroscopy system with the measurement unit for measuring an absorption signal from the sample. In particular, the terahertz beam can be reflected for multiple times through the sample in the measurement unit for enhancing the absorption signal. The present application discloses three non-destructive methods of measuring the absorption signal of the sample using the terahertz spectroscopy system.