A CFD software was used to research the characteristics of leakage flow and the processes of mixing between leakage flows and main flows on the downstream rotor for the labyrinth seal of a steam turbine rotor cascade.The performance of tip flow field with and without labyrinth seal was compared.The simulation results show that,the tip flow field is filled with complex vortex movement,which causes the full dissipation of the kinetic energy in leakage flow.The kinetic energy is dissipated more fully by increasing the number of labyrinth seal teeth.The mixing loss of leakage and main flow can be reduced effectively by increasing the labyrinth seal,and the effect of mixing can be weakened by increasing the number of labyrinth seal teeth.