The azimuth resolution improvement problem is solved via a coherent combination of synthetic aperture radar(SAR) images with the quasi-non-overlapped Doppler bandwidth. Prior to the spectra combination, SAR images should be co-registered, while phase biases induced by topography, atmospheric propagation delays and baseline measurement errors should be calibrated. However, the coregistration accuracy suffers from large Doppler decorrelation caused by the quasi-non-overlapped Doppler bandwidth. Furthermore, the method used to estimate phase biases from interferogram of azimuth pre-filtered SAR image pairs will fail when there is no overlapped spectrum. The fringe simulation and maximum sharpness optimization are adopted to deal with the problems. Accordingly, a novel algorithm to coherently synthesize SAR images is presented. The experiment with the Terra SAR X-band(TerraSAR-X) satellite data validates the performance of the presented method. The azimuth resolution improvement problem is solved via a coherent combination of synthetic aperture radar (SAR) images with the quasi-non-overlapped Doppler bandwidth. Prior to the spectra combination, SAR images should be co-registered, while phase biases induced by topography, Moreover, the method used to estimate phase biases from interferogram of azimuth pre-filtered SAR image The fringe simulation and maximum sharpness optimization are adopted to deal with the problems. The experiment with the Terra SAR X-band (TerraSAR- X) satellite data validates the performance of the presented method.