For signal light spot based on the acousto-optic deflection with RF signal changing in the coherent optical detection system，the relationship between spots structure and mixing efficiency is studied by using spots overlapping physical model，effects of optical axis deviation，active photosensitive surface radius，and local oscillator energy match are analyzed on coherent detection efficiency. The computer simulation and experiment test show that，using uniform energy distribution spots as local oscillator，this is very helpful for coherent mixing efficiency improvement within a certain range drift of signal light spot，when the signal light axis deviation is in the range-0.5~0.5mm，the overlapping efficiency declines only by less than 5%. Through study of energy distribution and spots structure，the basis reference for subsequent applications can be provided.