The influence of injection parameters on combustion oscillation inside an ethylene-fueled scramjet combustor equipped with a cavity flameholder is experimentally investigated for Mach 5.5 flight conditions. Experiments focusing on the influence of the fuel equivalence ratio, the premixing distance, the injection angle, and the number of jets have been carried out. Under certain conditions, high-speed camera and schlieren images show that flame oscillation occurs between the fuel injectors and the leading edge of the cavity. It is also demonstrated that those factors mentioned above can significantly affect the flow-shock interaction which generates low flow speed region and the mixing status which correlates with combustion intensity closely. The strong interaction between combustion and the separated boundary layer downstream of the cavity forms a thermal throat inducing thermal chocking to cause flame flashback. The distributions of the flame front, flame propagation speeds and quasi-periodic oscillation frequencies obtained by the iso-luminosity contour method show distinct differences in different cases. The strong interactions of flow and shock, combustion and separated region can easily trigger flame flashback, and the flame front can be maintained around the injector for long time in cases of deeper penetration depth and longer premixing distance. A simplified combustion opening system model predicts that the flame under sensitive conditions may acquire an exponential amplification once thresholds of the thermal fluctuation are exceeded.