Abstract: A new criterion named interferential degree of background was proposed to estimate the quality of infrared small target image. It was described quantitatively by two indicators, one is ‘confusion degree of background upon target’ which represents the ability of the background providing false targets, the other is ‘shielding degree of background upon target’ which represents the ability of the background shielding the target. Experimental results show that the results estimated by this criterion are more consistent with the facts compared with traditional methods (SNR, signal to noise ratio), and they can provide more accurate information about the difficulty degree of detecting or recognizing the targets which is concerned by the researchers. Meanwhile, the criterion is valuable for quality estimation of small infrared target image which does not contain the information of the target.