This paper focuses on a novel squint spotlight SAR mode, where the PRI variation is employed to increase the range swath width, especially for high-resolution imaging. The spotlight SAR is developed to increase the azimuth resolution via steering the antenna beam to always illuminate the same area on the ground during the whole data acquisition interval. However, requirement of high resolution and large squint angle leads to large Range Cell Migration (RCM). Therefore, to ensure the scattered echoes along the azimuth to be completely received within the fixed reception window, the range swath has to be much narrower than the reception window. In order to increase the range swath, we can change the PRI along the azimuth to shift the reception window according to the variation of instantaneous slant range. This paper first derives the PRI variation scheme. Afterward, a modified time-domain Back-Projection Algorithm (BPA) is presented to implement the focusing. Finally, simulation results are given to validate the presented SAR mode and corresponding imaging processor.