Infrared imaging systems have many advantages, such as full-time employment, good concealment, high resolution, strong anti-interference ability, so they have become the military and civilian focus so far. In this paper, we explored to construct the evaluation system about detection performance of infrared imaging systems based on infrared physics theory and the action mechanism between infrared radiation and detector. First of all, infrared detection mechanism of point target has been investigated under long distance condition. We derived the solving formula of detection range and proposed the method of calculating the mutual position of target and detection system to reduce false alarm rate.Secondly, we constructed the evaluation system on detection performance of area target, and mainly combined the area distribution of target imaging on the photodetector, signal to noise ratio of target, the contrast between target and background. The detection range of area target was the minimum value of the three factors. The simulation results indicated that the constructed evaluation system of point target and area target can fully evaluate detection performance of infrared imaging systems.