Abstract: To achieve successful ignition and stable combustion in an ethylene-fueled scramjet, the hydrogen is applied as the pilot fuel to ignite ethylene at low flight Mach numbers. Flow characteristics, flame propagation characteristics and combustion stability have been investigated in a scramjet combustor via various strategies of fuel injections, such as fuel injection of single hydrogen, single ethylene and combinations of fuels. The inflow conditions are Mach number of 2.0, a total temperature of 953 K and a total pressure of 0.82 MPa at the entrance of scramjet combustor. Multiple non-contact optical measurements, including the schlieren, CH luminosity images and OH-PLIF, have been applied to detect flow structures and flame propagation along with the 10 kHz pressure transducers monitoring the pressure of the centerline on the top wall of combustor. The results indicate that without fuel injection, the internal flow of scramjet would oscillate at a dominant frequency of approximately 450 Hz. With fuel injection, the oscillation is suppressed when the fuel is injected upstream of the cavity and there is no effect on the internal flow when the fuel is injected downstream of cavity step. OH-PLIF images reveal that the flame of pilot hydrogen is unstable. The flame mainly locates in the middle and posterior of cavity when the pilot hydrogen is injected upstream of cavity and OH radicals repeatedly gather and disperse in the middle of cavity. The flame of hydrogen cracks in shear layer and there is no chemical reaction around cavity ramp, when the pilot hydrogen is injected downstream of cavity step. Meanwhile, the combustion is also unstable with combined injection strategy. When the pilot hydrogen is closed, the flame transfers from the middle and posterior of cavity to the cavity ramp, then stabilizes, indicating that the combustion instability of combined injection strategy derives from the combustion instability of pilot hydrogen.