Searching for targets in complex scenes like X-ray baggage screening is demanding. One of the facts that cause impaired search performance is the superimposed objects of X-ray images. In this study, we compared searching for threat items in superimposed displays (global and partial) and a sparse display. Using both behavioral and eye movement measures (fixation duration, fixation number, time to fixate on target, dwell time on target, direction and end position of initial saccade), we explored the effects of superposition on search course and search strategies. As expected, superposition affected accuracy rates and response times. Results of Exp.1 showed that searching in the superimposed display caused longer fixation durations, and the effect was pronounced throughout the search trial. In Exp.2, we found the search guidance time and target verification time were both elongated in the partial superimposed display. Subjects’ search strategies were also affected. Initial saccade was directed to the superposition area when the area was located near the fovea. In sum, the present study is the first to explore effects of superposition on search course and search strategies with eye-tracking technique. Results carry implications for improving security screening performance.