This paper deals with the azimuth resolution improvement problems via the coherent combination of the SAR images with nearly zero-overlapped Doppler bandwidth, which corresponding to large along-track baseline. Prior to the combination of the images, the phase bias contributions from the atmospheric propagation delays and baseline measurement errors should be calibrated. The interferogram of the azimuth pre-filtered SAR images are adopted in the classical method, whose performance falls down when the overlapped spectra comes to zero. In this paper, the maximum sharpness optimization is adopted to deal this situation. The proposed method has been tested by processing the real data. (4 pages)