A numerical study was carried out for the aero-optical effects of flow around an aircraft on laser beam propagation. Beam propagation is described by the paraxial scalar wave equations,solved by the phase-screen technique and the FFT method. Air density variation is governed by the complete Navier-Stokes equations which are solved by using the LU-SGS factorization technique. The numerical results show that the deflection and defocus effects would be significant due to the variation of flow field. Thus,the necessary compensation should be in consideration when use optical instruments on aero-craft.