A measurement algorithm based on Point Spread Function (PSF) was proposed to accurately measure radiation characteristics of infrared small targets which are tiny or quite far away from the IR imaging system. With the method, the PSF of the optical system was measured by using the slanted edge method under the premise of given target shapes. Then, the processes of imaging and sampling of the infrared focal plane arrays were modeled and the simulated image of the target on a detector was obtained. Furthermore, the radiance, size and center position of the target were estimated accurately by matching the simulated image and the actual image obtained from the IR imaging system. Finally, experiments were performed to verify this method. The obtained results show when the temperature of a circular target is 70 ℃ and the ideal imaging diameter is greater than 4 pixels, the maximum error of radiance measurement is 2.35% and that of the target size (area) measurement is 1.89%. The experimental results demonstrate that the method is feasible and is suitable for capturing radiation properties of targets or backgrounds, performance measurement of infrared systems, image restoration and small target positioning.