According to the difference between theory detection and battlefield detection ability of airborne IRST system, the relation between airborne IRST and operational requirement or detection ability was analyzed firstly. Secondly, considering the operational factors as relative entering angle, azimuth angle, pitch angle, environmental background radiation and so on, the models of detection ability in level and pitching angle on airborne IRST were built. Finally, the concept of detection probability envelope was put forward based on operational environment and the character of detection probability, and the model of detection probability envelope on operating range and the ability of airborne IRST and its probability were analyzed quantificationaly in different battlefield situations. It can be seen from the results of simulation that when meeting the demands of false alarm probability and SNR, the detection probability envelope has optimum detection point, which provides theoretical reference for guiding aircraft to change the situation and develop the operational effectiveness of airborne IRST. Meanwhile, it also provides theoretical support for simulating real battlefield environment and test in lab.