Abstract: In this study, the operating modes of the rotating detonation engine (RDE) under various inlet conditions were investigated numerically. It was shown that after the sudden reduction of the inlet total pressure, the resulting decrease of the mass flow rate would require the rotating detonation wave (RDW) to adjust its front height and propagation velocity so that it could maintain self-sustained propagation. However, if the inlet total pressure dropped significantly, the number of RDWs in the flow field would decline to match the new inlet conditions. This would lead to an adaptive mode switching process. The approximate matching relation between the characteristic parameters of the RDWs (wave number, propagation velocity, and front height), the geometry of the combustor, and the inlet conditions (mass flow rate) was verified to explain the operating mode switching process and the underlying mechanisms.