To carefully investigate the mechanism of thin light beam aberration passing through compressible mixing layer, double-exposure CCD camera imaging system and schliren system have been used to synchronously measure the flow field and aero-optical aberration. The distribution of density gradation can be obtained by use of schliren system. In addition, owing to adopting CCD camera imaging system, the aberration images of thin light beam can also be acquired. Moreover, adopting double-exposure technique can present the information of motion and shape of a large-scale structure, and then show its effect on optical aberration. A conclusion from the experimental results can be drawn that the existence of large-scale structure in flow field leads to the tilt and blur of light beam and the motion of large-scale structure results to the corresponding motion of light beam.