Highly squinted stripmap synthetic aperture radar(SAR) signal processing exits two difficulties, i.e., the severe range azimuth coupling caused by the large deviation angle and the realtime implementation of full aperture SAR processing. A new subaperture mosaic imaging algorithm for the highly squinted stripmap SAR is proposed. In this algorithm, the improved palar format algorithm (PFA) is used to realize the high precision imaging of highly squinted SAR. The full aperture realtime imaging is realized by the sub aperture image mosaic. Both imaging results of simulations and measured data prove the validity of the algorithm.