Abstract: A whole annular steady and unsteady numerical simulation is carried out on a low speed large-scale axial-compressor to investigate the relationship between tip leakage vortex breakdown and fluctuation and rotating instability. The results indicate that periodic variation in the pressure difference between the pressure surface and the suction surface of blade tip is induced by the tip leakage vortex unsteady breakdown at the near stall operating condition, resulting in a periodic variation of the leakage speed, which in turn causes the leakage vortex to fluctuate periodically. Observing from the circumferential space, the phase differences, which exist at the adjacent blade, bring out a rotating instability phenomenon with a stable wave number and propagation velocity. It has certain significance for knowing about the flow mechanism of rotating instability.