3D wavenumber domain imaging algorithm takes no approximation of the range course and owns high image reconstruction precision. The cross-track dimensional array length is much shorter than cross-track dimensional imaging swath in downward-looking array 3D SAR. In order to avoid wrapping during FFT operation, the echo signal should be zero-padded to the size of the imaging scene. Extortionate zero-padding operation causes high memory requirements and computation loads. Low zero-padding is required if Region Of Interest (ROI) is only dealted instead of the whole imaging scene which helps to boost the algorithm. In this paper, a method is introduced which accomplishes wave-propagation and along-track dimensional, wave-propagation and cross-track dimensional 2D imaging, then the ROI is determined based on the twice imaging results. Finally, the ROI is processed with 3D wavenumber domain imaging algorithm. The effectiveness of the introduced method is verified with experimental data.% 3维波数域成像处理方法对回波信号距离历程不做近似，成像重建精度高。机载下视阵列3D SAR跨航向阵列长度相比跨航向幅宽小很多，需将回波信号尺寸补零到成像场景尺寸以防止FFT时出现卷绕，过高的补零倍数给波数域成像处理带来内存需求和运算量的激增。如果能够仅对ROI (Region Of Interest)而非整个观测场景进行成像处理就能够极大程度降低补零倍数，提高该算法的时效性。该文提出的波数域快速成像方法首先在波传播-航迹向和波传播-跨航向完成两次2维成像处理，结合两次2维成像处理结果确定ROI，最后使用3维波数域算法对ROI进行3维精确重建。实验数据验证了该文算法的有效性。
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