Spotlight mode is a special mode of Synthetic Aperture Radar(SAR), which can achieve higher resolution image than the ordinary strip-mode SAR. Starting from the top-level parameters design of high resolution space-borne spotlight SAR system, combining with the detailed analysis of system design and simulation process, many influence factors including long migration distance, Pulse Repetition Frequency(PRF), antenna pattern on spotlight SAR system design are analyzed. This work can provide a reference for space-borne spotlight SAR top-level parameters design.