In this paper， based on the blackbody radiation theory and the test principle of the minimum resolvable temperature difference (MRTD)， the imaging quality evaluation parameter for infrared thermal imager， the quantitative relationship between infrared radiation difference and temperature difference of the target and background at different ambient temperatures is established. And then the ambient temperature influencing factor for infrared thermal imager is proposed. The results show that the MRTD of IR thermal imager with good non-thermal design and the ambient temperature influencing factor has the same trend with the change of ambient temperature， and the ratio of the two tends to be the same at different temperatures. This conclusion can be used to evaluate the imaging quality of infrared thermal imager under temperature stress.