Abstract: The operating range is an important parameter of the IR alarm system, and its value is decided by the characteristics of the alarm system, the target and the background. A research was done specially on the operating range of ground-based IR alarm system in clear sky. The traits of the transmission and radiation of the atmosphere were fully considered, and then the theoretic calculating model of the operating range was established. Combining the model and typical data, and considering the two situation of cover target and spot target, the effects of the atmosphere on the operating range were calculated for different target altitude and high angle. The results show that the target can be detected only when the target's high angle is larger than a certain value if the altitude of the target is fixed, and this angle decreases as the target attitude increases.