The characteristics of impulse signal synthetic aperture radar (SAR) imaging are analyzed in this paper. It is shown that the linear array model is an efficient interpretation to the azimuth integration of an impulse SAR. Back projection (BP) method can be comprehended easily with a linear array model. With respect to the algorithm realization in a field system, the BPIQ method with back projection to I, Q channels respectively and then evaluating the module of two integration's is presented. Computer simulations show that the imaging results of BPIQ are better than those of the ordinary BP algorithm.