Abstract: 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.