A modified model of operating range for cooled infrared detection system is presented. The limitation of traditional operating range model is analyzed,and the existing model is modified from three aspects including detectivity,signal noise ratio and aberration after considering the influences of cold diaphragm,background radiation and aberrant on operating range of the cooled infrared detection system. The operating ranges for two typical targets (missile and airplane) are calculated by using the modified model. The results show that compared with the traditional model,the modified model is more suitable for the actual situation and has a higher accuracy. This can provide a reference for the design and evaluation of cooled infrared detection system,and can also be used to direct the observation station arrangement for the infrared system.