The issue of oil-gas-water interface has attracted much attention in the oil industry. The spectral characteristics and optical properties of oil-gas-water interfaces of the sample cells with the thicknesses of 1 mm and 0.5 mm are investigated respectively by using the Terahertz-Time Domain Spectroscopy(THz-TDS). Bimodal response is observed at the kerosene-gas interface. When the THz-TDS is transformed to Terahertz Frequency Domain Spectra(THz-FDS), an oscillation effect is obtained. The oscillation phenomenon will be the most significant when the test center arrives at the kerosene-gas interface. Besides, the oscillation phenomenon in THz-FDS attenuates with the decreasing of the sample cell’s thickness. Such oscillation phenomenon will not appear in the THz-FDS for kerosene-water interface. Results indicate that THz spectroscopy can be used as a secure method to characterize the oil-gas-water interface in the transportation process of petroleum.