In order to accurately and quickly obtain the real motion and distribution of the complex internal flow field in the gearbox of a high speed train during its operation,the three-dimensional model of the driving gearbox of high-speed EMU was reasonably simplified,and the Moving Particle Simulation(MPS) was used for simulation analysis.Based on different instantaneous distribution states of oil in the gearbox,the influences of key parameters such as forward and reverse rotation conditions,oil immersion depth and oil properties on the internal flow were studied.The oil churning loss of gear pair under different working conditions was calculated,and the influence of different factors on the oil churning loss of gear pair was analyzed.It is concluded that when the pinion rotates counterclockwise,the oil reaches the meshing area faster,and there are more oil particles in the meshing area,which is conducive to the lubrication of the meshing area.When the pinion rotates clockwise,the oil is more likely to diffuse inside the gearbox.The increase of the oil amount may produce more high-speed particles and cause more power loss,and the oil viscosity significantly influences the oil amount flowing into the bearing housing and the power loss during the oil stirring process.