The diffraction from a target for a synthetic-aperture laser imaging radar is distinguished into three domains, and the methods of defocusing of telescope and spatial phase modulation by an additional plate are suggested to compensate the wavefront aberration of echo. In the Fresnel diffraction region the quadratic aberration of echo can be eliminated by the defocused or phase-biased telescope and a phase history for aperture synthesis is correspondingly generated, and in the Fraunhofer region the quadratic wavefront can be also eliminated by the defocusing or phase-biasing but no phase history. Nevertheless, in the Rayleigh-Sommerfeld diffraction the higher-order aberrations cannot be compensated.