The spectral characteristics of solid α-lactose are studied by using the attenuated total reflection Terahertz Time-Domain Spectroscopy(THz-TDS) technique. Firstly, the experiment is based on the traditional transmission spectroscopy technique, and two strong absorption peaks at 0.53 THz and 1.36 THz are obtained. In addition, taking two samples ofα-lactose with the doses of 1.8 mg+2.56 mg and 1.8 mg respectively the absorption lines are obtained by the attenuated total reflection THz-TDS system. Strong absorption peaks appear at 0.53 THz and 1.36 THz. Increasing the sample dose has no effect on the absorption intensity at 0.53 THz and 1.36 THz. Finally, taking two samples of α-lactose with the same doses, one is screened to 75 μm, and the other is not screened, the absorption lines are obtained by the attenuated total reflection THz-TDS system. Strong absorption peaks appear at 0.53 THz and 1.36 THz. When the particle size of the sample is reduced, the absorption intensity increases around 0.53 THz and 1.36 THz.