This study investigates the mixing enhancement mechanism and propagation characteristics of the detonation flow field of a Rotating Detonation Engine (RDE). Three-dimensional numerical simulations of a non-premixed ramjet-based RDE fueled by gaseous ethylene are performed in OpenFOAM for configurations with 15, 30, 45, and 60 orifices at a flight Mach number of 4. The results show that fuels with a stripped distribution are primarily mixed via tangential diffusion in the cold flow field. The configuration with more orifices has a better upstream mixing efficiency, whereas its downstream mixing efficiency, which is limited by the depth of penetration, is difficult to improve further. Backward Pressure Perturbations (BPPs) opposite to the propagation direction of Rotating Detonation Waves (RDWs) are produced by the reflection of the upstream oblique shock wave with the incoming stream and the hot release of local reactions after RDWs, which significantly affects the propagation mode and mixing. The RDWs propagate in the stable single-wave mode in configurations with 45 or 60 orifices and in the multi-wave mode in configurations with 30 orifices, whereas they fail in configurations with 15 orifices. Compared with that in the cold flow field, deceleration of the main flow, pressurization, and tangential velocity perturbation caused by the RDW substantially enhance the mixing efficiency. Moreover, the tangential velocity perturbations of upstream oblique shock waves and BPPs reduce the unevenness of the fuel distribution for the next cycle. This study reveals the mixing enhancement mechanism of RDWs and can contribute to the design of the injection scheme of the RDE.