Aiming at the big difference between theoretical and battlefield detection ability of airborne photoelectric system and the inadequacy of operation ability, the spatial distribution function of target reflective background radiation is obtained and the quantitative method is given. The target original radiation is studied and the models of level and pitch radiation intensity envelope are built. Considering the various operational factors, the equations of detection ability in level and pitching angle are given. The concept of detection probability envelope is put forward, and the relationship between operating range and detection probability is obtained and the envelope probability characteristic of airborne photoelectric system is analyzed quantitatively. The simulation results show that detection envelope changes with the change of target speed，azimuth and pitching angle. When meeting the demands of false alarm probability and signal noise ratio, the detection probability envelope has optimum detection point, which provides theoretical reference for developing the detection performance of airborne photoelectric system.