The Stripmap-mode SAR is able to image larger area during the same time than the Spotlight-mode SAR. There are always some ROIs which should be emphasized in the whole SAR image. But the signature from those scatters outside the ROI will decrease the contrast of ROI's image. The digitally-spotlight algorithm is able to filter the unwanted signature before imaging to achieve the equivalent purpose to spotlight-mode SAR and increase the contrast of the SAR image. The simulation results prove its validity.