The polar format algorithm(PFA) for bistatic SAR imaging is based on th e planar wavefront assumption which introduces wavefront curvature effect in the focusing process. There is space variant geometry distortion and defocusi n g effect in the focused image via bistatic PFA. Hence, the focused scene size us ing bistatic PFA is usually restricted. A new met hod is proposed to solve the problem of wavefront curvature compensation using t he digital s potlighting technology. Firstly, the digital spotlighting is preprocessed t o divide the original wide beam into several narrow beam phase history data, eac h of which corresponds to different small image patch. And then each narrow beam is compensated and imaged. Finally all these fo cused image patches are mosaiced together to get a full scene image. Theory and simulation results show that the method c an effectively compensate the wavefront curvature effect and enlarge the focused scene size for bistatic PFA.