The thickness of flow field around the platform is defined based on aero-optical effect, the relationship between the thickness of flow field around the platform and the characteristic time of the aero-optic effect for the mean flow is investigated, and the rough estimation formula of characteristic time is given. Using large computational fluid dynamics software (SAED), the change of airborne optical flow field around the platform with respect to time and the thickness of flow field is computed, and change rule of aero-optic effect for the mean flow is researched. The calculation results verify the reliability of the rough estimation formula. It is shown that characteristic time of the aero-optic effect for the mean flow field is proportional to the ratio between thickness of flow field and the plane speed, which is in agreement with that of turbulence effect. In the given model, characteristic time is about 5 ms, and the characteristic frequency is less than 300 Hz. The root mean square of optical-path difference induced by the aero-optic effect for the mean flow field varies around a mean value with time. The ratio of variance to mean value is about 8%.