A threedimensional unsteady numerical simulation was adopted to research the structure and the frequency spectrum characteristic of the leakage vortex in the tip labyrinth seal cavity.The variation law of radial height with clearance of vortex core in each cavity of labyrinth seal was analyzed.The simulation results show that, there are two stable dissipative vortices in the tip seal cavity of steam turbine,the cavity vortex and the shroud wall vortex.With the increase of tip clearance,the radial height of vortex core is decreased and the radial height of vortex core is increased.From the inlet to the outlet of the tip labyrinth seal,the vortex in the cavity becomes violent with the flow,the frequency is increased and the amplitude of the fluctuation becomes larger.The shroud wall vortex changes alternately in different cavities.The front tooth wall vortex is the most unstable type of vortex in the tip labyrinth seal cavity,in which the pressure fluctuation is the most severe.