Abstract: Bandwidth synthetic technique provides a fine range resolution for the advanced airborne SAR system. In strip-map mode, in order to get an as good azimuth resolution as the range, the precise motion compensation is important. This paper studies several key issues of the two-step motion compensation method in the bandwidth synthetic system. In the first compensation step the precise Inertial Measurement Unit (IMU)/GPS data is used, the real target distance and the ideal airborne line are always unknown. In this paper, the corner reflectors are used to estimate these values. And in the second step, an improved Phase Gradient Autofocus (PGA) algorithm is used to compensate the residual time-mutative phase error. Real data testifies that after the two-step compensation the azimuth resolution better than 0.25 m is obtained finally.