Spotlight SAR is an efficiency way to obtain high resolution radar imagery. The non-stability of the aircraft motion during the long synthetic aperture time can make the imagery suffer the sever defocus, and there is phase error in the range direction in the high resolution SAR system. This paper presents a novel signal processing method to get high definition resolution spotlight SAR imagery, which can make an effective estimation on the aircraft position error, the phase error in range direction and the residual envelop error. The experiment results show that the proposed approach can get the high quality SAR image from raw data.