This paper focuses on quantitative detection performance for point target of single-sampling and over-sampling systems and presents an imaging chain simulation model based on the sampling model of sub-pixel, which depicts detection for targets with various heights and sizes under given resolution conditions. Single-sampling and over-sampling with the same IFOV, over-sampling of two times IFOV were taken into account. Performance of the three sampling systems was compared and analyzed from the following four aspects: target signal’s acquisition capability, background signal and clutter level, detectability and measurement characteristics. Results are listed as follows: over-sampling of the same IFOV is conductive to the acquisition of the target signal and has better detectability, while that of two times IFOV suppresses target signal and goes against detecting target with low SCR; over-sampling has inhibitory effect on background signal and clutter; over-sampling collects apparently superfluous radiation intensity and the error has to be corrected.