The terahertz (THz) absorption spectra of citric acid (CA) and its monohydrate (CA·H2O) were acquired at ambient temperature through terahertz time-domain spectroscopy, covering the frequency range of 0.5-3.5 THz. Subsequently, we analyzed these spectra by density functional theory (DFT) to provide theoretical insights. The investigation focuses on the underlying origins of the observed THz absorption peaks, emphasizing the agreement between theoretical predictions and experimental observations. Besides, we predicted the dehydration temperature of CA·H2O by employing thermogravimetric analysis and differential scanning calorimetry (TG-DSC). We confirmed the theoretical interpretations by comparing the THz experimental data of CA·H2O without water molecules. We found that the measured peaks (<2THz) on CA and CA·H2O originate from the intermolecular forces. The characteristic absorption features at 1.29 and 2.63 THz of CA·H2O result from the interactional forces of H2O-CA molecules. Finally, a quantitative analysis was conducted on the mixed samples of CA and CA·H2O, focusing on their changes in the amplitude of the characteristic absorption peak (1.29 THz) at different sample concentrations. The correlation coefficient R2 of the fitted data exceeds 98%, laying the foundation for the quantitative detection of CA and CA·H2O.