The absorption peaks of three cinnamic acid derivatives (CADs): p-coumaric acid (PCA), trans o-coumaric acid (OCA), and 4-fluorocinnamic acid (4-FCA) were measured using terahertz time-domain spectroscopy (THz-TDS) in the range of 0.5-3.5 THz. To identify the origins of the THz absorption peaks in the three samples, the density functional theory (DFT) was employed along with the vibrational mode automatic relevance determination (VMARD) method. Additionally, the molecular force field energy decomposition analysis (EDA-FF) was used to analyze the forms of weak intermolecular forces in the molecular systems. Visualization analysis was conducted through the visualization of molecular dynamics (VMD) using atom coloring to study the contribution types and strengths of weak intermolecular forces by atoms in the molecular systems. This combination of THz-TDS, DFT, VMARD, and EDA-FF methods not only effectively distinguishes organic molecules with structural isomers or structural similarities but also provides valuable reference data for uncovering their biochemical functionalities.