Active near-infrared imaging system used for traffic monitoring and regional security monitoring is an important application of night vision system. The function range of night vision system is an important performance indicator and the main basis for system design. A method for evaluating the function range of system is proposed according to the needs of the design and analysis of active near-infrared imaging system. The method can determine the detection capability of system on the target at a given distance. Based on the calculation of the distribution of irradiance on the target at a given distance according to the atmospheric transmission theory, the irradiance is simulated utilizing the principle of equivalent lighting at a short distance. Then the detection capability of the system on the target at a given distance is evaluated according to Johnson criteria and imaging theory. The corresponding tests are carried out, and the tested results are compared with the actual outdoor measurement results. The method is applicable for general optical imaging systems.