Currently, there is no standard method of evaluating the performance of the gas leak infrared imaging detection system. The evaluating criterions vary greatly and are deficient in aspects of completeness and accuracy, such as noise equivalent temperature difference, noise equivalent concentration path length, and minimum detectable leak rates. Minimum resolvable gas concentration (MRGC) is a latest proposed parameter for evaluating the performance of a passive gas leak infrared imaging detection system, which takes full advantage of the comprehensive evaluation capability of the temperature resolution and spatial resolution of the minimum resolvable temperature difference (MRTD) model. The MRGC takes into account the environmental and gas state parameters, the size of the gas plume and other factors which influence the MRGC measurement. However, the MRGC measurement system is complicated and many state parameters need to be controlled, especially the wide range and dedicated gas concentration meters are required. Therefore, the mathematical model of MRGC is derived and established. By comparing the principles and measurement methods of the performance parameters, MRGC and MRTD, a novel MRGC equivalent measurement evaluation method is proposed, on condition that the minimum resolvable radiation differences are equal. Using ethylene and ammonia as the target, the equivalently measured results of MRGC are obtained. The results show that the MRGC increases with the spatial frequency increasing and the smaller the temperature difference is between the gas and the background blackbody, the faster the MRGC increases. What is more, when the spatial frequency is fixed, MRGC increases with the gas temperature approaching to the background temperature. The background temperature varies asymptotically, which means that if the gas temperature equals the background temperature, the system cannot detect the gas four-bar pattern, no matter what the gas concentration is (here, the maximum gas concentration is 1 million ppm under normal pressure.). The directly measured and equivalently measured results of ethylene are in good agreement within errors of less than ±20%, and the maximum error is 18.26% at a spatial frequency of 0.214f0, which demonstrates the feasibility and effectiveness of the method. Because the equivalent measurement method only needs the traditional MRTD measurement results and the gas infrared spectrum database, it is simple and reliable, which is very significant for the study and application of the gas leak infrared imaging detection systems.