Abstract: Supersonic cooling gas film is a key technique applied to hypersonic imaging guided vehicle. For ratio of static pressure (RSP) is a major factor affecting the development of supersonic cooling film, it has great engineering significance to carry out a systematic research on the RSP. In this view, flow visualization tests of supersonic cooling film at different RSP levels based on nano-tracer planar laser scattering (NPLS) were carried out in a M6 hypersonic wind tunnel. Through the analysis of transient flow visualization images, the development process of the shear layer between the hypersonic mainstream and supersonic jet was studied. With the analysis of fractal dimension and intermittency, the distributions were, respectively, obtained of fractal dimension along the flow direction and intermittency factor along the normal direction, and the effect of RSP on turbulence development was analyzed. The evolution of supersonic gas flow field mainly includes four stages which can be characterized by laminar shear layer, disturbance occurrence, mixing of "mainstream-jet" shear layer and jet boundary layer, and formation of "large boundary layer". RSP has obvious influence on the wave structures, the film thickness and the turbulence degree of the flow field. The overall thickness of the gas film is positively related to the static pressure ratio along with the "condense" for a under-match RSP and the "inflation" for a over-match RSP. The film thickness of under-match state and match state increases slow first but then fast, meanwhile it is just opposite under over-match state. The turbulence breaking factor of under-pressure state and match state is generally smaller than that of the over-pressure state in the forepart of flow field, but it increases rapidly along the flow direction. Moreover, the match state has the largest turbulence breaking factor and the highest degree of turbulence level compared with the other states.