During a radiation measurement mission, the target size may be too small or quite far away, which results in small infrared target images that can not be measured directly as an extended source. However, the target image tends to occupy several pixels, and therefore can not be treated as a traditional point source. Aiming at this problem, a small target radiation characteristics measurement method, based on the traditional measurement method of point source is proposed. The imaging characteristics of small targets are studied by experiments and theoretical analysis for the purpose to explain the difference between extended source and small target. A formula for small target radiation measurement is deduced by subtracting the background radiance from the target area. Finally, a mid-wave infrared imaging system is used for small target radiometric experiments, and the results indicate that the proposed method is effective.