Based on the technology of Nano-based Planar Laser Scattering(NPLS), the ambient flow field of a cavity was studied in the research, in which the ethene fuel was injected perpendicularly in the upper part of a cavity placed in a scramjet engine model. The study focused on the mixing process of the fuel jet and its mainstream, the comparison of the changes in the fuel jet penetrability under different injection stagnation pressures, as well as the effect of the cavity geometry parameters on the mixing process of the fuel jet and its mainstream. Through the study, the effect of the injection stagnation pressures on the mixing of the fuel jet and its mainstream was found mainly in two aspects: the penetrability and the diffusion of the fuel jet. As for the fuel jet, this study indicated that the main phase of the mixing process with the mainstream begins when the eddies come forth and ends when the eddies break up. Moreover, this study showed that a closer location of the injector helps to produce a better mixing of fuel jet and mainstream and a higher-quality transportation of the fuel jet to the cavity. In addition, the influence of the cavity configuration on the fuel jet primarily starts from the stage of the breaking up of the large-sized eddies.