The evaluations of aero-optic effects for subsonic airborne electro-optical (EO) system from shear-layer cavity flow are presented. Compressibility effects in the shear layer are measured by convective Mach number 0.8, the geometry of cavity is L/H=6.7 and the average cavity temperature is T=230 K. Numerical simulation of this unsteady two-dimensional flow using the commercially available software CFD code has also been carried out and it proves that Strehl ratio improves with wavelength increasing;however, the dispersion angle is complex. The wavelength effect on the system performance is also theoretically validated and then an optimum wavelength model λ*=2πσt is given.