Taking designed labyrinth seal of a high speed EMU gearbox as the research object. The numerical simulation of the internal flow field of labyrinth seal is carried out by using the Fluent software,the pressure,speed,velocity vector and turbulence distribution cloud of the internal flow field of the labyrinth seal are obtained. The sealing mechanism and process of the gearbox labyrinth seal is analyzed. The results show that the fluid flows through the sealing gap when the pressure is reduced,the pressure is reduced very little.When the fluid flows out of the sealing gap into the cavity is the pressure equalizing process. After entering the cavity,the turbulivity of the cavity is the largest and the pressure is the smallest. As the distance from the center of the cavity increases,the speed increases and the pressure increases. The distribution characteristics of the flow field in the labyrinth seal under different pressure ratios are analyzed,which indicated that the leakage increases with the increase of the inlet/outlet pressure ratio,and the pressure ratio is not linearly related to the leakage. The simulation results can be used as reference for the design of gearbox labyrinth seal.