Three sugar compounds were selected as the research objects because of their importance to the human body. Their absorption spectra were measured in a wide frequency range by high resolution terahertz time domain spectroscopy and Fourier transformation infrared spectroscopy systems. It has been found that glucose has the characteristic absorption frequencies of 1.10, 1.30, 1.45, 1.79, 1.88, 1.97, 2.08, 2.40, 2.55, 2.70, 2.84, 2.96, 3.24, 3.64 and 4.23 THz, fructose has the characteristic absorption frequencies of 1.09, 1.33, 1.65, 2.14, 2.62, 2.97, 3.24, 4.75, 6.97, 7.35, 7.98, 8.36, 9.16, 9.32, 9.53 and 9.73 THz, as well as galactose has the characteristic absorption frequencies of 2.21, 2.33, 2.70, 2.82, 3.17, 3.42, 3.93, 4.51, 5.07, 5.96, 6.60, 6.91, 8.03, 8.71 and 9.01 THz. By analyzing the experimental results of samples mixed from glucose, fructose, galactose and polyethylene quantitatively, it has been known that at the measured characteristic absorption frequencies, absorption increases linearly along with increasing the mass fraction of glucose, fructose and galactose. Furthermore, both the glucose and fructose have a common fingerprint frequency in 2.96 THz, glucose and galactose have a common fingerprint frequencies in 2.33, 2.70 and 2.82 THz, fructose and galactose have common fingerprint frequency in 8.00 THz, and all three kinds of compounds have common fingerprint frequency in 3.20 THz. Because the three samples have the same molecular formula, the 3.20 THz characteristic absorption mainly comes from intramolecular interaction, which represents the same chemical bonds or groups of isomers. The difference in characteristic absorption frequency is due to the molecular structure and inter-molecular interaction, which represents the difference between isomer structure and inter-molecular vibration mode. It can be predicted that glucose fingerprint frequencies would be tested in 4.70, 5.30, 5.60, 5.98, 7.03, 7.85, 8.26, 8.71 and 9.01 THz in development. Based on Density Functional Theory, CASTEP software was adopted to optimize the structures and calculate the characteristic absorption frequencies of samples. The theoretical simulation results are in agreement with the experimental ones. This result shows that CASTEP is feasible to compound molecules research in the THz range.