For synthetic aperture radar(SAR) imaging of ship targets, it's difficult to generate high quality images because their motion characteristics are blind in most cases. Aiming at this problem, this paper presents a processing scheme for estimating the radial velocity based on the digital spotlight processing. This method is implemented after the coarse-resolution wide area image is obtained. Then the digital spotlight technique is carried out to extract the interested ship target from the original image and reform the motion compensation processing according to the real center of the extracted target. This processing greatly improves the SAR image quality of the moving target, while its accurate radial velocity is determined by finding the minimum value of the image entropy curve obtained from a searching procedure. The ship-target simulations fully prove the effectiveness of the technology.