Currently, the minimum resolvable temperature difference (MRTD) test of a thermal imager is mainly undertaken indoors. To meet the testing requirements of MRTD in other environments, a portable MRTD tester is proposed. This paper describes the development principle of the instrument, optical optimization design, mechanical design, and environment adaptability design, as well as the design points of the blackbody and other components. At present, this instrument has been used in the MRTD test of thermal imagers in outdoor environments and shows accurate results.