In this study, hydrated and anhydrous polymorphs of 3,4,5-trihydroxybenzoic acid (3,4,5-THBA) were characterized by Fourier transform infrared (FT-IR), Raman and terahertz (THz) spectroscopic techniques at room temperature. The density function theory (DFT) calculation with B3LYP method is applied to simulate the corresponding optimized structures and vibrational frequencies. The different vibrational modes are assigned by comparison between the experimental and theoretical spectra of two polymorphs. There is obvious difference within the molecular vibrational modes of hydrated 3,4,5-THBA and those of anhydrous one. The results show that the interactions between water and 3,4,5-THBA molecules make molecular configurations significantly different within hydrated and anhydrous polymorphs. This work offer the experimental and theoretical benchmark for identifying pharmaceutical polymorphs and also further studying the intramolecular and intermolecular interactions with spectroscopic technologies.