The shafting structure of helicopter transmission system is complex, then its relative position changes will affect the transfer characteristics of the whole transmission system. Using tail drive system of a certain helicopter - a pair of spiral bevel gears system with perpendicular shafts as the study object, the variation characteristics of the gear pair meshing force under relative position changes of shafting are studied. By establishing a dynamics model of the pair of spiral bevel gears system with relative position changes of shafting, and combining with the multi-body dynamics software,the variation characteristics of the gear pair meshing force are simulated and analyzed. The results show that the average meshing force decreases with the increase of offset position changes, and increases with the increase of inclined angle position changes. The inclined angle position changes have more serious influence on gear meshing characteristics than the offset position changes. This research can provide a theoretical basis for condition monitoring of helicopter tail drive system, and it is important to improve the stability of the helicopter transmission system and ensure its safe and efficient operation.