Due to the difference between theory detection and inbattlefield detection ability of airborne IRST (Infrared Search and Track) system, the method of evaluation for detection system was put forward. Firstly, the theory aiming at operational capability of airborne infrared detection system was analyzed, especially the betterment of NEFD (Noise Equivalent Flux Density) operating range model, and the analysis of functional relationship between operating range and detection probability. According to the operation mode of IRST system, the computational formula of physical detection probability and target identification probability was deduced, and the target identification probability changing with operating range as well as speed was analyzed. Finally, the test platform of IRST system detection probability was set up and the experimental evaluation steps were made, and a simple and reliable method of evaluation and calibration was proposed. Simulation result shows that the operation range limitation of IRST system can reach 60km.Testing by semi-physical simulation，the rationality of the model is verified through the experimental conclusion.