The simulation of the mean flow over airborne laser fairing as well as its aero-optic analysis is presented. The simulation reveals that the aero-optic effect with respect to the mean flow strongly depends on the beam direction, and the aberration is mainly low-order effect. The thickness of mean flow around the platform is defined and estimated based on aero-optical effect. A rough estimation formula about the thickness of flow field around the platform, the platform speed, and the characteristic time of the aero-optic effect, is brought about. The calculation results verify the reliability of the rough estimation formula.