Infrared Search and Track (IRST) system is an optical imaging detection equipment used widely onboard the aircrafts.A method for evaluating the detection distance of IRST system is presented here,which is adaptive for use in the lab.Taking the target irradiance as the index,and by using a measurement system to accurately simulate the target irradiance,the evaluation of IRST detecting distance is transformed into the measurement of target detection sensitivity.We made test to multiple IRST products in the lab under the same conditions,and carried out the evaluation to the detecting distance through analogy analysis.The result shows that the method is simple and efficient.