In order to make clear the mechanism of oil churning loss of gearbox and to further improve the efficiency of gear transmission,the factors that effects on oil churning losses are studied. The finite element model of oil fluid in gearbox is established in terms of the condition of the gear transmission under oil-immersed lubrication. The variation of oil splash and pressure on meshing point are analyzed under different speeds and directions of gear rotation,the gear resistance moment and the oil churning loss are calculated. The final results show that,under medium and low speed transmission condition,the change of gear rotating direction has greater influence on oil-pumping volume on the meshing area,oil-throwing volume of the outer circle of the gear,pressure on meshing points,and oil churning loss. The above conclusion may provide theoretical basis for later research on energy saving strategy of gear transmission.