Aiming at the test results of minimum resolvable temperature difference (MRTD) of a multi-band common aperture long focal length reflective infrared imaging system deviating from design value under laboratory conditions, starting with the definition of MRTD, the influences of observation frequency response, visual angle, sampling phase transfer function and other factors affecting MRTD test results were analyzed in detail. Secondly, the influence of sampling phase on infrared system static and dynamic MRTD test results was emphasized. It was demonstrated that static MRTD test value would deviate from the design value when spatial frequency was in the range of $0.6\;{f_N}$～ $0.9\;{f_N}$. Finally, combined with the test data, it was proved that sampling phase transfer function was the main factor affecting the MRTD of infrared imaging system.