To achieve the accurate temperature measurement in fields by infrared thermal imagers, the second calibration of atmospheric transmission was researched. A calibrating model for temperature measurement by infrared thermal imagers in a far distance was established, and an infrared thermal imager and a standard expanded area blackbody were used to do the second calibration of atmospheric transmission. Firstly, the setting temperature of the standard blackbody was used to calibrate the secondary calibration coefficient of atmospheric transmission. Then, assuming the emissivity of interested area of a object to be known, the secondary calibration coefficient was used to calibrate the measuring values of a unknown radiation object and to implement the accurate measurement of radiation temperature of the radiation object. Experiments show that as the setting temperature for the blackbody increases from 50 ℃ (the second atmospheric transmission is approximately equal to 1), the second calibration coefficient of the atmospheric transmission decreases rapidly in the temperature of 50~100 ℃, and decreases more and more slowly in the range of 100~200 ℃. Gradually, it is close to a constant value 0.7. These results can provide references for the accurate temperature measurement by infrared thermal imagers in fields.