Ali in Tibet, 5100 m above sea level, is one of the most suitable locations in the world for infrared spectral observations. The atmospheric transmittances at Ali Observatory and Mauna Kea Observatory were calculated by MODTRAN using radiosonde data. The results were 0.848 and 0.789 respectively which indicated better conditions at Ali Observatory. A self-made instrument with a 320x256-pixel HgCdTe infrared focal plane array and a 7.5-cm diameter telescope was utilized for the actual measurements. Without the help of standard stars, the on-site and real-time atmospheric transmittance can be obtained as 0.831 by fitting the relation between the measured atmospheric radiation intensity and the zenith angle based on radiation transfer equations. This paper firstly reports the atmospheric transmittance in theM' band (4.605-4.755 mu m) at the 5100 m-altitude Ali observatory by actual measurement. It shows that the high-altitude Ali observatory with sufficiently low water vapor content is suitable for observation in the mid-infrared bands.