Abstract: Experiments are conducted in a small transonic wind tunnel to investigate the impacts of the high-speed flow on the transmission features of light beam. Aberrated wavefronts of light beam travelling through the flow field in the wind tunnel test section with nominal Mach numbers 0.7, 2.0, 3.0 are measured by a Shack-Hartmann wavefront sensor based optical system. Some interesting results including the peak variation (PV), the root of mean square (RMS) and the far-field focal spot are obtained, which suggest that the influence of high-speed flow field increases with the increased flow speed, and the related aberration of wavefront grows rapidly especially when the Mach number is greater than 3.0. Thus it is essential to eliminate the severe disturbance of the flow field in the test section