Because of Wavenumber Domain Algorithm (WDA) using a more accurate signal form and range cell migration correction technology, it shows high accuracy in squint Synthetic Aperture Radar (SAR) data processing. However, the conventional WDA cannot fully utilize the complete Two-Dimensional (2-D) spectrum support region under high-squint condition, resulting in poor target focusing effect. In this paper, we propose a novel coordinate transformation-based WDA for high-squint SAR imaging. In the proposed method, the squint SAR echo is firstly transformed into the 2-D frequency domain, and the “uniform compression” of the entire data is completed by multiplying the reference function. Then, based on the principle of coordinate transformation, the tilted spectrum is converted into a rectangular form. Finally, residual focusing is achieved by a modified Stolt interpolation. Compared with traditional WDA, the proposed algorithm corrects the inclined spectral support region and ensures the imaging quality. In addition, the range and azimuth profiles of its point target impulse response function (IRF) are orthogonal, which facilitates the calculation of resolution and the extraction of profiles. Experimental results based on simulated and real data validate the proposed method.