A kind of auto-scanning down-looking synthetic aperture imaging ladar (SAIL) is proposed, which in construction uses two coaxial orthogonally polarized beams in transmitter and polarization interference based self-heterodyning detection in receiver and in principle adopts the wavefront transformation to generate illumination wavefronts with a hyperbolic spatial phase difference. Therefore, the wavefront scanning from the relative movement of target produces automatically the necessary linear phase modulation in the orthogonal direction of travel, and the quadratic phase history in the travel direction as well. The image is reconstructed by virtue of the Fourier transform with the Doppler quadratic compensation in the orthogonal direction of travel and the conjugate quadratic phase matched filtering with the crossed coupling compensation in the travel direction. The system is very simple in structure. Optical modulator isn′ t used. The initial phase synchronization for linear phase modulations doesn′t exist. The time delay of target doesn′t exist, too. And the advantages of the original down-looking SAIL remains such as the availability of big receiving aperture and big optical footprint, and the capability of suppression of phase interferences from atmospheric turbulence and others. The suggested SAIL is of practical potentials to applying for spaceborne and airborne observation or inverse-SAIL radar under every workable distance and speed.