The lubricating oil distribution and the internal pressure variation during gear engagement in the highspeed train unit gearbox were analyzed by Pumplinx software.The VOF twophase flow model based on fluidstructure interaction theory was established by using RNG k-ε turbulence model and moving mesh model.The flow field was simulated and the pressure of key positions was monitored.The effect of different gear parameters on the gearbox internal flow filed was analyzed.The results show that the vortex phenomenon appears in the gear engagement area and lower left of driven gear.The engagement inlet zone has the positive pressure fluctuation and the outlet zone has the negative pressure fluctuation,and the pressure fluctuation of the outlet zone is stronger than that of the inlet zone.The vent hole pressure fluctuation is strong and irregular.The gearbox pressure fluctuation is stronger when increasing the gear speed and the tooth width.It is conducive to obtain a uniform distribution of pressure in internal flow field through reducing the speed and tooth width reasonably.The distribution of the gearbox internal twophase flow and the variation law of pressure field can be obtained by simulation analyzing and monitoring the key positions,which can provide theoretical basis for rational design of gear box structure and rational allocation of lubricating oil.