Abstract: The image position of space-borne SAR is calculated based on range-doppler principle and then the scatter coefficientof terrain is simulated according to experientialformula.The(density)of simulated image points in mountain is not symmetricalowing to the difference between the interval of DEM and the resolution of SAR image,and side-look character of SAR.Four DEM corner points are used to interpolate the simulated SAR image.The space-borne SAR is rectified(after) thatthe simulated image is matched with the real SAR image.The method is demonstrated by simulating RADARSAT image and rectifying the real SAR image.