Testing problems for minimum resolvable temperature difference (MRTD) is discussed, which is an important parameter of infrared imaging systems. At first, traditional testing methods and principles for MRTD are introduced and the shortcomings are analyzed. And then, two objective testing methods and the intelligent testing method developed on this basis are discussed. At last, two improved alternative testing methods such as MTDP and triangle orientation discrimination (TOD) are introduced, and the advantages and disadvantages are analyzed respectively.