In this paper, an atmospherical attenuation calibration model for external infrared imaging, is established based on the recalibration of atmosphrerical transmittance with a standard blackbody and an infrared imager. Initially, the infrared camera undergoes calibration in a laboratory setting. Then, the least squares fitting of measured results is employed to determine the responsivity of the infrared imager and the fixed bias of the sensor, which arises from dark current. After that, a calibration model of atmopherical attenuation is formulated. When using infrared imager for external experiment, the output of display and blackbody radiation brightness are obtained by varyinging the distance of testing, as well as the temperature of blackbody. Through incorporating the relative spectral responsibility coefficient of detector, the corrected value of atmospherical transmittance at a certain position can be calculated. Notably, the deviation value between corrected atmospherical transmittance and simulation is less than 2%.