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 fulllink of the space-based detection system,including scene clut-ter,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 ofthe detection system. Finally,taking atypicaldetection 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.