Serious range-azimuth coupling and spatial variability of imaging parameters limit the focusing depth of conventional synthetic aperture radar (SAR) imaging methods for maneuvering high-squint frequency modulated continuous wave -SAR. To expand the effective imaging area, an imaging method based on time-frequency-domain perturbation is proposed. First, the slant range model based on data recording parameters is optimized to remove Doppler frequency shift and secondary coupling caused by in-pulse movement. Then, the spatial variability of range cell migration (RCM) is corrected by constructing the time-domain perturbation function to achieve unified RCM correction. In azimuth processing, the linear spatial variability of the first- to third-order Doppler parameters and the second-order space variation of azimuth modulation frequency are removed by the time-frequency perturbation filtering functions. The focus quality of the nonscene center is effectively improved. Finally, the effectiveness of the algorithm is verified by both the simulation data and the measured data.