The terahertz (THz) spectra of chlorothalonil have been investigated from experimental spectrum and theoretical simulation in range of 0.4-3.0 THz. Based on terahertz time-domain spectroscopy (THz-TDS), the absorption and refraction spectra of chlorothalonil were obtained. To analyze the experimental spectrum, making up for the deficiency of isolated-molecule analysis, the structural changes and vibrational absorption spectrum of chlorothalonil were calculated in solid-state form, then analyze the source of absorption peaks. The results clearly demonstrate that the solid-state theory simulation is in good agreement with experiment where isolated-molecule simulation is not able to reproduce the spectral features, and the solid-state theory simulation complete the assignment of eleven absorption peaks. The observed spectral features of chlorothalonil molecule mainly originate from intra-molecular interactions of Cl-C-C, N-C-C and crystal packing vibrations in crystalline unit cell.