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For the Doppler parameters coupling and spectral aliasing in the hybrid sliding spotlight mode of the spaceborne–airborne bistatic SAR (SA-BiSAR), a frequency-domain imaging algorithm based on echo preprocessing is proposed in this article. The article utilizes the idea of spectral analysis (SPECAN) technology to remove the spectrum aliasing by echo preprocessing, without subaperture partition and subaperture stitching operations, which reduces the computational burden. On this basis, the Legendre orthogonal polynomial expansion method is used to achieve a high-accuracy approximation of the SA-BiSAR slant range function. Combined with the method of series reversion, the accurate two-dimensional (2-D) spectrum of the point target is derived so that the high-quality decoupling of the 2-D spectrum is achieved. Finally, the compensation functions are designed to complete the focusing imaging in the frequency domain. The simulation results of point targets and distributed targets show that the imaging algorithm proposed in this article has good focusing performance and can improve the imaging quality of scene edge points.