In this letter, a novel imaging algorithm of downward-looking linear array 3-D synthetic aperture radar (SAR) is presented. To improve the resolution in cross-track direction, a multiple-signal-classification algorithm has been used. However, the computational cost is unattractive. To cover the shortage, fast Fourier transform (FFT) is utilized to roughly focus the targets in cross-track direction in this letter. Then, the range of peak searching can be considerably reduced. In addition, the backscattering coefficient can be directly obtained by FFT. On the other hand, since the scattering centers are always correlated in a real SAR system, the estimated covariance matrix is singular. To address the problem, the nearby spatial smoothing method is proposed. The array vectors of the nearby range and azimuth units are used to reconstruct the estimated correlation matrix. The effective aperture of the array can also be kept.