α-lactose hydrate is an important hydrate, and the effect of its bound water on the structure of lactose molecule has been one of the hot topics in the related fields. By setting the temperature step, the terahertz kinetics of α-lactose monohydrate at low temperatures was investigated using terahertz time-domain spectroscopy technique, and the temperature dependence of the two characteristic absorption peaks of α-lactose monohydrate at 1.2 THz and 1.3 THz was found. In order to further investigate the generation mechanism of the three absorption peaks of α-lactose monohydrate, the vibrational analysis of the absorption peaks was carried out based on density functional theory for single-molecule and unit cell theory calculations, respectively. The results show that the absorption peaks of α-lactose hydrate originate from intermolecular interactions between water molecules and lactose molecules and the vibrations of lactose molecular skeleton, and it is also shown that unit cell calculations are more accurate in predicting the vibrational modes than single-molecule calculations.