Abstract: Aimed at spaceborne sliding spotlight synthetic aperture radar(SAR) imaging mode, the hybrid factor was analyzed and amended along the range direction combined the geometry model firstly. Based on the analysis, the Doppler bandwidth of both single target and whole scene were researched, and the validity of the two-step approach to overcome azimuth spectrum aliasing was proved by mathematic derivation. Moreover, the limitation of two-step approach was illuminated by thorough research. Namely, the azimuth overlapping will appear in image domain without high pulse repetition frequency. A extended three-step focusing algorithm was presented to resolve the disadvantage of two-step approach. After azimuth data focusing, a deramp operation was adopted to finish the resample in azimuth with a selected deramp factor, by which the azimuth overlapping in image domain was overcome. And, the azimuth time extension caused by deramp operation was also researched. Finally, the simulation results justify the superiority of the extended three-step algorithm.