Abstract: Radiometric calibration is very important for radiation characteristic measurement of the infrared system. The indirect calibration system was adopted, which was equipped with the collimator collected with the area blackbody as the extended radiant source. Firstly response model of the infrared focal plane arrays and the response characteristics related with the blackbody radiation and integration time were analyzed. Then through the least square fit method, response function of pixel respon gray value versus target radiation and integration time was demonstrated. Finally, the response function at different ambient temperature was compared with the former function, which further verified the effectiveness of the equation. This method can simplify the calibration process and save time. Besides, the calibration equation can be used to conduct quantitative analysis on impact factors of the calibration, and measure radiance in a wide dynamic range.