Imidazole and its isomer pyrazole in the terahertz band have unique spectral features that can be used in their identification. To further investigate the formation mechanism of terahertz spectral features, the molecular structures of imidazole and pyrazole are modeled and optimized in this study based on the density functional theory. The vibration characteristics corresponding to different characteristic absorption peaks are determined considering the potential energy distribution. Moreover,the weak intermolecular interactions are qualitatively and quantitatively analyzed via graphical analysis of interaction region indicator, energy decomposition analysis based on force field and topology analysis of electron density. The results show that imidazole and pyrazole have different molecular vibration characteristics owing to differences in the dispersion interactions and quantity and strength of the hydrogen bonds, resulting in significant differences in the frequencies of their characteristic absorption peaks in the range of 0.4-2.4 THz. Additionally, the formation mechanism of the characteristic absorption peaks is related to the differences in the vibration modes and weak intermolecular interactions. Therefore, this study provides an important reference for identifying nitrogen-containing heterocyclic pesticide raw materials and acquiring microstructural information.