Abstract: Based on deep study of conventional range-doppler (R-D) algorithm in Synthetic Aperture Sonar (SAS), a improved R-D algorithm applied to squint looking as well as side looking by abandoning the Presnel assumption is derived in this paper. Secondary Range Compressing (SRC) is avoided and imaging effect is improved remarkably in squint-looking use. Moreover, the steps in the algorithm are expressed only by Doppler center and Doppler rate, which can naturally realize motion compensation combined with auto-focusing steps. At last it is concluded that the modified algorithm is effective by squint-looking SAS simulation.