The apparent average temperature difference and critical dimension of actual target and background were acquired through retrieving infrared target image.The retrieved apparent temperature difference and the measured MRTD curve were combined to determine the corresponding spacial frequency.And,a quantitative relationship between the target infrared image and the probability of detection and recognition was established with Johnson criteria and target transfer probability function(TTPF).The method avoids operater′s subjective judging,so the result is more impersonal.This method was experimentally validated in evaluating the detection and recognition probability of IR system.