Abstract: Fixed coefficient FIR has been applied to conventional SAR azimuth preprocessing But in practical usage, the severe azimuth modulation may arise when the motion of the carrier is not ideal In this paper, an adaptive azimuth preprocessing method is put forwand, which can greatly overcome the azimuth modulation of SAR signal Since this method can a meliorate the SNR of the SAR system, the auto-focus performance will be improved greatly The result is satisfactory