The flow field analysis of the metro gearbox is the basis of the research on the lubrication of the output bearing. The structural parameters of the gearbox cavity have a very important influence on the lubrication of the gears and bearings. The CFD numerical simulation model of splash lubrication of helical gears in the metro gearbox was established by using the computer fluid simulation software CFX and the immersion entity method, and the speed and volume fraction of lubricating oil at the inlet of the bearing oil collecting box at the output end under different working conditions were obtained. The influence of radial distance, axial distance and guide rib width on the volume fraction and velocity of lubricating oil in the plane of oil collecting box was analyzed. The results show that the volume fraction of lubricating oil in the plane of the oil collecting box is negatively correlated with the oil velocity at the rotating speed from 1 800 to 3 600 r/min; under a certain width of the oil guide rib, increasing the axial and radial distances will increase the volume fraction of lubricating oil in the plane of the oil collecting box and reduce the plane oil velocity of the oil collecting box;when the axial distance and radial distance remain unchanged, with the increase of guide rib width, the volume fraction of lubricating oil increases and the oil velocity decreases.