Since Infrared Focal Plane Arrays (IRFPA) have become the main stream in the development of infrared detectors, the use of IRFPA parameters to evaluate the comprehensive performance of an infrared detection system is very important. The operating range equation of an infrared detection system represented by Noise Equivalent Temperature Difference (NETD) is deduced. Considering the influence of background radiation and target imaging dispersion effect on detection, the operating range of a system for a point target under different visibility conditions is calculated. The result shows that the better the visibility is, the farther the operating range of the infrared detection system is. The variation of the detecting capability of the infrared detection system with NETD is analyzed. With the increase of NETD, the operating range of the infrared detection system for the target decreases remarkably. The research result is of referential value to the design and performance index evaluation of infrared detection systems.