The calibration data are meaningful in measurement of infrared radiometric characteristics for target and its background. They also play an important role in IR imaging guiding simulation test application. The field radiometric calibration methods for infrared detecting system with fixed focal length are analyzed at first. Three kinds of methods were briefly introduced and compared, and the applicability of different methods was pointed out. In order to compare the characteristic change of three methods, short distance radiometric calibration, a certain distance radiometric calibration and radiometric calibration using of extended area blackbody and collimator were used in this research. And then the experiments by different radiometric calibration methods were investigated. The short distance radiometric calibration results show that it isn′t suitable for infrared reconnaissance and warning detecting system. The certain distance radiometric calibration results show that it needs further calculation of environment radiation, path radiation and path transmission. Then, the environmental parameters is loaded in MODTRAN soft ware to predict environment radiation, path radiation and path transmission. But it′s difficult to obtain very accurate calculating results. To measure the target radiation characteristics, radiometric calibration using of extended area blackbody and collimator is necessary. The research is useful for technical innovation of the out-field radiometric calibration of infrared detecting system. It also can be applied to system design of target characteristics measuring system.