The geometric structure of the radial labyrinth seal of highspeed train unit gearbox was analyzed.To explore the radial labyrinth seal mechanism and the influence of relative engagement depth(ratio of engagement depth and tooth height) on the sealing performance,the internal flow field and leakage of the radial labyrinth seal was simulated by Fluent.Leakage under different fin engagement structures was studied.The results show that when the relative engagement depth is from 0.1 to 0.6,the labyrinth ventilation effect is enhanced to cause the increasing of leakage with the increasing of the relative engagement depth of radial labyrinth seal.When the relative engagement depth is 0.6,the fluid speed thoroughfare is existed in radial labyrinth and the sealing performance is the worst,while the sealing performance is the best when the relative engagement depth is 0.1.The labyrinth sealing performance is decreased with the increasing of the fin angle and the tip length.Seal fin should be sharpened by reducing the angle and the tip length in engineering application and the seal fin has the smallest angle.