Aimed at the problems that as for missile-borne SAR at a large-angle dive movement stage, the echo signal has serious coupling and space-variance due to its complex flight characteristics to make the traditional focusing algorithm invalid, an improved frequency domain imaging algorithm of polar format algorithm (PFA) is proposed. Firstly, a range model of large-angle dive missile-borne SAR is established, and the range history is expanded in Taylor series. And then the high-precision two-dimensional spectrum of echo is derived by the series inversion method. Finally, the high-order cross-coupling terms are decomposed in the two-dimensional frequency domain. Moreover, a new two-dimensional interpolation mapping function is derived, greatly improving the performance of focused image. Compared with traditional PFA, the proposed algorithm is more suitable for large-angle dive missile-borne SAR, and is valid. 