To meet online automatic infrared camera minimum resolvable temperature difference (MRTD) field testing requirements, a novel method for detecting MRTD was presented. In hardware aspect, the use of the single blackbody temperature control technology not only reduced the volume and quality of testing equipment but also guaranteed the accuracy of the temperature difference control between blackbody target and environment background, and by combining serial interface communication technology with CCD technology, the cooperation of the temperature control and image acquisition was achieved; In software aspect, the use of optimized algorithm could precisely extract the target and background region, and calculate the MRTD using gray difference between target and background region. The experiments show that the error is less than 0.02 ℃ compared with the results observed by eye. The testing equipment has simple structure, small volume, convenient operation, it can realize the automatic test of MRTD under field condition.