Abstract: Passive ranging measurement of infrared search and track（IRST）system was introduced, the error of ranging was analyzed and the decision condition for the identifying target was presented. Moreover, the precision of passive ranging though triple-station IRST system was analyzed based on combat operation. The result indicates that 2 km site arrangement can guarantee the passive ranging accuracy better than 8% within range of 22 km, and better than 15% within range of 40 km. Omnidirectional, long-distance and high-precision passive ranging could be achieved in this triple-station IRST system.