Aiming at acquisition probability with human in the loop, the terminal searching process of infrared imaging missile and the features of human-in-the-loop seeker were analyzed. The calculation models were built for the horizontal and longitudinal instantaneous field of infrared imaging view. The condition turning to another side search was investigated, including excessing the maximum search sector and over boundary of target area. The criterion of target discovery was presented, which integrated distance constraint and azimuth constraint. According to flow process, the factors which influenced acquisition probability was simulated, such as missile course, start searching distance and length of target area side and so on. Results show that acquisition probability excesses the minimum value 21% when searching along diagonal line of target area; the optimum start searching distance is different as the length of target area side changes, and if the length is less than 7 kilometer, the acquisition probability is greater than 0.85; improving the coverage of infrared imaging seeker or speeding up angular velocity of operator turning ray axis will increase acquisition probability.