Aiming for long-distance optical detection of weak aerial targets, we in this paper propose a method of performance characterization and key indexes matching design for a space-based detection system. Firstly, we analyze the main factors that affect the optical detectability of weak targets under complex environment based on the full link of the space-based detection system, including scene clutter, spectral radiation scale and geometric scale of detection system, etc. We then combine the above influencing factors to establish a performance characterization model which represents the relationship between the image signal-to-noise ratio of target and the above-mentioned influencing factors, that is, the performance representation model of the detection system. Finally, taking a typical detection scene as an example, by studying signal-to-noise ratio of target under different scene clutter and multi-scale coupling characteristics, the optimization of detection spectrum and its matching design with spatial resolution are proposed, which can provide theoretical basis and scientific guidance for the design of the space-based detection system, demonstration of system indexes and optimization of information processing algorithms.