Aimed at the problems that large range migration and severe rangeazimuth cross coupling remain in returning echo under condition of the highlysquinted synthetic aperture radar imaging (SAR) mode, the conventional polar format algorithm (CPFA) is modified and a method for highlysquinted SAR is proposed. According to the actual echo data storage form, the algorithm establishes a slanted data model of the highlysquinted SAR, and designs a twodimensional interpolating function of the slant range plane through the expansion of the twodimensional Taylor series in the range and azimuth directions, effectively improving the depthoffocus. Moreover, the modified polar format algorithm (MPFA) is more suitable for the undulating terrain because the projection of real data onto ground plane performed in CPFA is avoided. The simulation and real data experiments show that the proposed approach is valid.