In order to find the optimum threshold noise ratio (TNR) of the airborne infrared search and track (IRST) system, the operating range model based on the signal to noise ratio (SNR) is adopted to combine with the function relationships of false alarm probability, recognition probability together with SNR and thus the probability model of detection point target of the airborne IRST system is established. The relationship between the recognition probability and the operating range, the target speed and the oblique angle is mainly analyzed. Combining with the infrared radiation characteristics of aircraft, the mathematical detection probability model of airborne IRST system is established. The influence of the change of target azimuth and pitching angle on the operating range is simulated and analyzed under different TNRs. According to operational requirements, it is concluded that the range of the optimum TNR is from 8 to 10. The results provide references for increasing the detection efficiency of airborne IRST system in practical applications.