This paper investigates a centrifugal compressor and analyzes a critical phenomenon that an upper limit exists that the restart rotational speed of the auxiliary power unit（APU） cannot surpass. We obtain the numerical features of flows running through the compressor. Based on this， the forces imposed on the rotor blades and the variation trends of the corrected mass flow rate are dissected. These analyses demonstrate that （1） windmill’s state parameters obtained through interpolation in speed characteristic lines， like the mass flow rate， the pressure ratio and the temperature ratio， are accurate and reliable； （2） the main driving area is the backward-curved part of the rotor blades when the incoming flow thrusts the compressor； （3） the rotor outputs torque at a rotational speed of 5% or lower of the designed value， and restores the operation on the flow as the speed rises to 15% or higher of the designed value， resulting in an increase of the mass flow rate and the flow pressure； （4） the trends that the relative mass flow rate follow as the flight condition and the compressor rotational speed change can help to explain the upper limit of the rotational speed during APU airborne ignition.