Abstract: The stability of antenna pointing is researched from the viewpoint of the effects of antenna pointing instability of spaceborne synthetic aperture radar(SAR) on image quality. Based on uniformity weighting antenna, the location and amplitude of the paired-echo resulting from instability of antenna pointing are derived in detail. And the effects of paired-echo on peak sidelobe ratio and integrated sidelobe ratio of impulse response function are analyzed. Moreover the Doppler center frequency error due to instability of antenna pointing is studied. According to the offsets of SAR imaging dependant on the Doppler center frequency error, the required stability of antenna pointing is given. At last the computer simulation is made to verify the analysis.