When the target is at high temperature and far from the imaging system, its image on the detector array occupies dozens of pixels and the output grayscale is easy to saturate. As a result, typical process methods for point-source target and extended-source target are not applicable any more. To overcome these problems, a measurement method is proposed measure high-temperature infrared small targets, and the effectiveness of the proposed method has been validated by experiments in the outfield. First of all, the radiometric calibration formula of measurement system in high temperature range is built, and the influence of the collimator transmittance on the calibration model is corrected. Then, according to the fact that the target energy diffuses dozens of pixels but the total energy that reaches the detector array remains unchanged, a measurement method is proposed to measure the characteristics of high-temperature infrared small targets. Finally, a Φ600 mm infrared radiation characteristic measurement system is employed to measure the radiation of high-temperature infrared small target in the outfield. Experimental results indicate that, in consideration of the atmospheric transmittance and path radiance, the proposed method can achieve high-accuracy measurement for remote high-temperature infrared small targets, and the effectiveness of the proposed method has been verified.