Abstract: For Frequency Modulated Continuous Wave Synthetic Aperture Radar (FMCW-SAR) at sliding spotlight mode, only time domain algorithm is available. That is because an azimuth window function suitable for each point target echo in the scene can not be obtained, which yields numerous operation. A novel fast echo simulating algorithm in range-Doppler domain is presented to solve the problem. Under some approximations, the algorithm successfully forms the azimuth window function by adding an azimuth linear frequency modulated signal and the phase information of the echo is obtained in range-doppler domain, which increases greatly the calculation speed while the simulating accuracy remains relatively high. The more the point targets are, the better performance in speed compared to the time-domain algorithm is achieved. S他和rmesfully SSsssimulations verify the effectiveness of the presented algorithm.