Based on the demand of site selection for large infrared telescopes, an infrared radiation measurement device is developed to measure the M' band (4.605-4.755 μm) atmospheric infrared radiance and extinction characteristics at Lijiang and Chengjiang observatories. The data of temporal and spatial variations of atmospheric radiation is analyzed by Allan variance method and atmospheric radiative transfer equation, and the influence of temporal-spatial variation of radiation on infrared astronomical observation is discussed. The results show that the radiative fluctuation in the low frequency region is large, Allan variance increases exponentially with integration time, and the fitting parameters of Allan variance for Lijiang and Chengjiang observatories are 0.794 and 1.238, respectively. From zenith to 60° zenith angle, radiance of Lijiang and Chengjiang observatories increases by 68% and 72%, and the transmittance decreases to 0.46 and 0.52, respectively. Infrared astronomical observation needs chopping, and the best chopping frequencies at Lijiang observatory of the single pixel, 2×2 Binning and 4×4 Binning modes are 0.030, 0.070, 0.144 Hz, respectively. Allan variance, atmospheric extinction, and the best chopping frequency can be used to guide the site selection and design of large infrared telescopes.