Abstract: This paper analyses the geometry of sliding spotlight FMCW-SAR, sets up the echo model and derives the two-dimensional spectrum of its echo signal. According to its signal characteristics a subaperture wavenumberdomain algorithm is presented, which corrects the Doppler Frequency Shift Effect caused by the motion within the sweep. The algorithm can acquire the images with azimuth resolution better than that obtained in the stripmap case by the subaperture coherent recombination and with an imaged area larger than that achieved in the spotlight operation by adjoining the subaperture images. Rationality of the echo model and the effectiveness of the proposed algorithm are verified by the simulation results and analysis.