Severe flow oscillation induced by low-mass-flow of inlet directly threatens flight safety. The unsteady simulation on the low-mass-flow induced instability phenomenon of the Bump inlet under over-speed operating conditions at Ma₀=1.8 is conducted. The effects of different throttling levels on the inlet is studied. The results show that when the inlet mass flow is reduced to 48% of the through-flow mass flow rate， the inlet enters a periodic surge state. Based on the flow characteristics， the instability process can be divided into three typical stages， i.e.， terminal shock expelling out of duct， backflow development， and the internal flow re-establishment. Driven by the high pressure inside the tube， the terminal shock propagates forward， with its movement speed initially decreasing and then increasing. As the terminal shock merges with the lip shock， it triggers large-scale flow separation. The interaction between this separated flow and the incoming freestream generates unsteady vortical structures， which disrupt the inlet shock system. In the later stage of the backflow development phase， significant reverse flow is observed near the inlet， and a zero-velocity interface forms in the vicinity of the outlet section. Subsequently， the pressure inside the duct gradually decreases， allowing the internal flow field to be reestablished. As the second throat diameter decreases （corresponding to a further reduction in mass flow）， the maximum pressure ratio inside the duct increases from 6.61 to 6.84， and the normalized backflow propagation distance ratio increases from 2.43 to 3.33.