Abstract: Taking the blade tip clearance in high-pressure stage of steam turbine as an object of study, numerical simulation was performed on unsteady leakage flow in the clearance using k-ω sst turbulence model and PISO algorithm, so as to analyze the incidence, trajectory and intensity of the leakage vortex. Results show that the tip leakage flow is of the cyclic and unsteady type; the incidence, trajectory and intensity of the leakage vortex vary with the time and tip clearance height; the effect of tip leakage flow on main stream exhibits a regular variation, from weak to strong, and then from strong to weak; the strength and incidence of the leakage vortex reach their maximums at T/4; and at T/2, the tail-edge shedding vortex of stator blade mix with the leakage vortex in the front of suction surface of rotor blade, creating a new vortex system, while the leakage vortex in the rear of suction surface of rotor blade mix with the separation vortex at its boundary, leaving the suction surface of rotor blade.