Abstract: The flow instability triggering process at stall condition of NASA Rotor 37 tr-ansonic compressor rotor at low Reynolds number was investigated by unsteady numerical simulation.The result shows that the effect range of boundary layer radial vortex is extended,the boundary layer separation blockage zone due to radial vortex is assembled at the blade tip,as the stall condition develops,the boundary layer separation blockage zone moves to the blade leading edge,and combines with blockage zone due to shock and tip leakage flow.So the blade tip passage is fully blocked,and compressor flow instability is triggered off.