A method is established to calculate aero-optical effects.It is valid for nonuniform flow fields.A three-dimensional spatially developing mixing layer is simulated by large eddy simulation(LES).Favre-filtered Navier-Stocks equation is discretized with a high order compact difference scheme.Transition from laminar to turbulence is simulated successfully.And it is compared with that of direct numerical simulation.Aero-optical effects caused by mixing layer is calculated to study relation between flow field and imaging distortion.It shows that imaging distortion is the severest at transition region.