The catalpol, which can be isolated from the Chinese traditional herbs Rehmannia glutinosa as an effective and active ingredient, is an iridoid glycoside with many pharmacological functions. The molecular structure and the infrared (IR) spectrum of catalpol were calculated using density-functional theory (B3LYP) at 6-311G** level, and the theoretical frequency was scaled by 0.96. The vibrational modes of IR spectrum were assigned and compared with that of the experimental data. The calculated IR spectral features from DFT are in good agreement with the experimental ones. The results indicate that DFT is an useful method for the structure optimization and IR spectrum calculation of the iridoid compounds, which can be isolated from natural resources and often with difficulty. The theoretical calculation results of IR spectrum can provide useful information for the prediction of the theoretical property and investigation of the structure-activity relationship of the iridoid compounds.