In order to explore the feasibility of vibration sensing behind barriers by terahertz interferometry, a terahertz interferometer was built based on a Michelson interferometer and using 214.58 μm CO2 laser pumping gas THz source. Small vibrations behind a thin cardboard caused by knocking a target mirror was measured by the terahertz interferometer . With some data processing techniques such as phase analysis and spectrum analysis, vibration time-histories and time-frequence distribution were resolved. The maximum of measured peak-peak amplitude is 17.54 μm and the minimum is 7.98 μm. The calculated peak velocity is about 2.7 mm/s. The maximum and minimum of vibration frequence are 58 Hz and 21 Hz, respectively. The results show that terahertz interferometry is a convenient novel technique that senses vibrations behind barriers, and can effectively overcome disadvantages of conventional vibration sensors.