Numerical simulations were performed on tip unsteady flow field of a transonic compressor rotor with different work mass flows. The results show that no oscillation occurs in tip flow field of rotor under high mass flow condition

which can be regarded as steady flow field for analysis

and the oscillation of tip leakage vortex is intensive under low mass flow conditions. Furthermore

periodical interference within the tip leakage flow between adjacent blade passages emerges. As a result

a phenomenon called "rotating instability" caused by the oscillation of tip leakage vortices and its circumferential propagation

is found in stationary frame. The dominating cell number of "rotating instability" flow field is nearly 40% of the blade passage number

and the cell size in circumferential direction is 2-3 pitches.