Abstract <p>Porous media seepage flow is the active flow control method used to reduce heat and skin friction in high-speed vehicles, but effective measurement techniques for mixing the seeping gases with the incoming boundary layer are lacking. By premixing approximately 20% acetone vapor in the cooling gas and employing the acetone planar laser-induced fluorescence (PLIF) technology, flow of the seeping gas film within the boundary layer was visualized. A correlation between the relative intensity of PLIF image grayscale and the gas film mixing rate is established. Experimental results showed that the seeping gas film layer remains initially laminar in the Mach 3 laminar boundary layer; with the lower injection rate, the film layer develops slowly and maintains a longer laminar state. As the injection rate increases, the film layer thickens significantly along streamwise direction on the porous wall. After reaching a certain thickness, instability develops, leading to intensified mixing with the incoming boundary layer downstream and the formation of large-scale mixing structures. The position of instability moves upstream with increase in the injection rates, indicating that the higher film injection rates tend to induce boundary layer instability and premature transition. When the injection rate <i>F</i> &lt; 0.2%, the diffusion rate of the seeping gas film into the outer boundary layer is low, and the film maintains a high concentration at the bottom of the boundary layer. With the higher injection rates, the mixing ratio increases and diffuses outward, with a slight decrease in the normal concentration gradient of the film along the wall. For a given injection rate, the diffusion range of the seeping gas film continuously increases but does not exceed 5 mm in thickness. The study shows that the acetone PLIF technology can effectively achieve fine visualization and quantitative analysis of the mixing flow structures of seeping gases within supersonic boundary layers.</p>