Experiments were performed in a direct-connect supersonic combustion test facility to simulate scramjet combustor acceleration in high altitude. The combustor inlet flow Mach number increased from 2.40 to 2.94, and the flowrate and total temperature simultaneously changed with Ma. The combustor has two cavities and fuel jets. The fuel used is room-temperature liquid kerosene RP3. Mode transition is seen to occur in the first cavity during acceleration, and pressure fluctuations occur in the transitions from both the ram mode to the transition mode and from the transition mode to the scram mode, indicating that the mode transition process is unstable. When the mode transition occurs upstream, the downstream ram mode has the effect of eliminating instability; therefore, the engine’s overall thrust performance is stable. When the downstream is also in the transition mode, the thrust fluctuates, indicating that the mode transition is an unstable process.