Abstract: In order to investigate the flow field characteristics of an ethylene fueled rectangular scramjet in different combustion modes, three-dimensional steady Reynolds averaged Navier-Stokes simulations of the flowpath were employed on the basis of direct-connect experiments for four different equivalence ratios. The numerical and experimental results were compared. The distinguishing criterion of the combustion modes for this configuration was chosen. The regularities of the sidewall pressure and one-dimensional average Mach number distributions were discussed. And the detailed characteristics of the shock structure, the flow separation, and the combustion were analyzed. The results indicate that the simulation results are in excellent agreement with the ground-test data. Multiple reflections of the oblique shock waves and expansion fans result in the wall pressure fluctuation, and the shock system is mainly affected by the flowpath structure for Case Cold. In the scramjet-mode operation, the influence of the shock produced by the interaction between the flow and the injectors on the flow field is obvious, the pressure rise is anchored downstream of the injector, and the flameholder cavity is full of three-dimensional separated structures. In the dual-scramjet-mode operation, the oblique shock train induced by shock-boundary-layer interactions dominates the flow field structure, and the shock system is weak in the combustor. And some separation occurs closely behind the shock train leading edge in the corners of the isolator whereas the separated region reduces in the cavity. In the ramjet-mode operation, the shock features are similar to that in the dual-scramjet-mode operation. The separation regions expand in the isolator corner and reduce in the cavity with the upstream propagation of the shock train. Some combustion may occur in the isolator with the upstream propagation of the shock train for the dual-scramjet-mode and ramjet-mode operations, while in the scramjet-mode operation the combustion is conducted just in the cavity and expander, and the chemical reaction and high temperature distributions are more concentrated.