TOPSAR (Terrain Observation by Progressive Scans) is a novel spaceborne SAR mode for wide-swath observation, which solves the scalloping problem of ScanSAR though steering antenna beam in azimuth direction. The relationship between azimuth resolution and cycle time of TOPSAR was derived. Furthermore, the constraint of parameters was analyzed and the flow of engineering parameter design was presented. Finally, a TerraSAR-X like sample was provided and a simulation was conducted with the design result in SBRAS (SpaceBorne Radar Advance Simulator). The results show the effectiveness of the design.