Abstract: Wind tunnel free-stream disturbance is a key factor affecting the boundary layer transition. In this paper, a set of non-invasive Focused Laser Differential Interferometry (FLDI) measurement systems sensitive to density perturbation is constructed and calibrated. An experimental study on the environmental characteristics of wind tunnel free-stream disturbance is carried out for the JF8A wind tunnel of the Institute of Mechanics, Chinese Academy of Sciences. By experimentally selecting incoming flow conditions with varying Mach numbers and unit Reynolds numbers, the characteristics of freestream perturbations in large-scale shock tunnels were determined through density pulsation measurements using FLDI. The experimental results show that: at the same Mach number, the density pulsation amplitude gradually increases with the increase of unit Reynolds number, but the normalized amplitude gradually decreases; at the same unit Reynolds number, the slope of the free-stream tumble is more gentle for the state with high Mach number, which indicates that more energy is retained in higher frequency bands. The normalized RMS value of the density pulsation decreases as the unit Reynolds number increases, this feature follows the same trend as the variation of Pitot total pressure perturbation level with Reynolds number for a typical hypersonic noise wind tunnel. Finally, the intermittency of the free-stream disturbance environment under various conditions was investigated using short-time PSDs(power spectral densities), and the results showed that the intermittency gradually decreased with increasing unit Reynolds number.