Abstract: In order to solve the image geometric distortion problem of missile-borne synthetic aperture radar(SAR) when using spectral analysis(SPECAN) algorithm during the down flight stage, a novel approach based on coordinate mapping principle is introduced in this paper. According to the geometry of missile-borne SAR, echo model of point target is established. Then the coordinate mapping principle which is suitable for SPECAN algorithm during the down flight stage is analyzed in detail. Through mapping each point of the SAR image coordinate system to the spatial distance coordinate system, we finish the correction. Extensive simulation results corroborate the validity of the proposed method. Furthermore, compared with other correction algorithms, the error analysis indicates that the proposed algorithm of this paper is more accurate and superior.