Using dimensional analysis and experimental technique, the aero-optical effect for a compressible mixing layer around optical side window is investigated. According to the theoretical analysis, we find that time-averaged boresight error increases with the increase of convective Mach number (Mc). Moreover, we also present the relations between time-averaged boresight error (BSE) and Reynolds number, shear stress at interface between mixing layer and free stream, free stream velocity ratio and density ratio. The experimental results from a thin beam light passing through compressible mixing layer verify the theoretical result between time-averaged BSE and Mc.