Based on the analysis of the scanning procedure of single-sampling and over-sampling systems, we proposed a novel scanning image simulation method. The target energy gathering, clutter suppression, and dim target detection of single and over-sampling systems were performed in 3 different scenes, mild-cloudy, severe-cloudy, and cirrus-cloudy. Analysis and experiments indicate that over-sampling system has better performance at target energy gathering, clutter suppression, and dim target detection.