As the previous clutter metrics have such problems as extracting the weight value from the fixed experience and taking no consideration of the human visual response, they cannot measure the background clutter accurately. We propose a new clutter metric, in which the optimal response threshold is introduced to simulate the adaptability of the human eye to the clutter response. The optimal response threshold is also used to evaluate the effect of the background similarity on the target acquisition performance so as to measure the clutter. The proposed metric is tested by the Search_2 database. The testing results demonstrate that the target acquisition performance prediction based on the clutter metric is in accordance with the subjective data. The mean square error of the target detection probability is 0.0835, and the correlation coefficients are 0.7124 and 0.7444; the square mean error of the target false alarm rate is 0.0691, and the correlation coefficients are 0.7874 and 0.6753; the square mean error of the target search time is 3.2321, and the correlation coefficients are 0.7630 and 0.7710.