In order to find the cause of oil loss of an aero-engine intermediate bearing oil collector.The oil-air two-phase flow in the oil collector is studied by the VOF model, a numerical method of multi-phase flow.The flow field and oil movement in the oil collector are analyzed, and the effect of inner ring rotate speed on oil collecting efficiency is given. The results show that the higher the rotating speed, the lower the oil collecting efficiency. The flow in the oil collecting annulus is mainly in the circumferential direction, and the asymmetric swirling vortices appeared on the meridional surface of the annulus. The reason for the oil loss is that the oil is split into two branches of oil flow from the entry port and collided with the inner surface of the cage, one branch to the inside of the bearing, and the other to the leak port. The lubricating oil accumulated near the inner side of the oil dam is absense of force that drives the oil to the inner direction of the bearing. When the oil accumulated at the oil dam reaches a certain amount, the oil overflows the oil dam and leaks into the bearing cavity.