In the condition of beam point steering, spaceborne spotlight SAR antenna model and imaging model were found. The math analysis confirmed that beam point steering resulted in echo amplitude being modulated and caused infinite pairs of echoes. So beam point steering would deteriorate image quality. Besides, based on the analyses of tradeoff relation of beam point steering ratio, imaging area azimuth width and image dynamic extent, the fundamental principle of designing beam point steering ratio was presented. Finally, point target simulation was given and simulation result shows the validity of theoretical analysis.