The atmospheric infrared radiance was measured at Ali, Delingha and Huairou observing station using a self-made measurement system in the infrared M′(4.605-4.755 μm) band. Based on the blackbody calibration and the radiative transfer equation, a simplified relation between the effective output value, the average zenith atmospheric transmissivity and the zenith angles can be obtained. Atmospheric infrared radiation of different zenith angles at three Astronomical Station were measured and scanned, and the above formula was used to fit the average atmospheric transmissivity of the M′band. The measurement results show that the weighted average values of the atmospheric transmissivity in the three places are 0.805, 0.758 and 0.650 respectively, with the fluctuations of 0.081, 0.250, and 0.073 respectively. The average transmissivity simulated by MODTRAN software was respectively 0.851, 0.805, 0.615, which was close to the results from the measurement. The error analysis shows that the propagation error decreases with the increasing effective output value. The theoretical error of indirect measurement method was analyzed to be less than 10%. This paper provided a on-site and real-time measuring method of the atmospheric infrared transmissivity independent of meteorological data.