The helicopter intermediate gearbox operates under splash lubrication, and the bearing will produce metal particles due to insufficient lubrication or wear. If the metal particles signal at the bottom of the gearbox fails to absorb and alarm in time, it will lead to the helicopter failure. Therefore, based on Computational Fluid Dynamics (CFD) theory, this paper established the solid-liquid-gas three-phase flow model of the intermediate gearbox, using Fluent 18.2 software. Based on the Euler-Lagrange coupling calculation method, the accessibility of metal particles was calculated and analyzed by applying the RNG k-ɛ turbulence model and dynamic grids and the VOF-DPM coupling model simulation, which achieved good consistency with tests. The results show that the motion of the metal particles is mainly dominated by their own state upon initial entry into casing. After the gear completely stirs the lubricating oil, the motion state of the gear dominates, and the randomness and disorder of the movement are fairly great. The trajectory will abruptly change after bouncing off the casing wall. All metal particles in the gearbox tend to move centrifugally around the gear, and the average velocity of the metal particles shows a nonlinear negative correlation with the distance from the gear. The accessibility of metal particles in the gearbox is weak, and the metal particles will deposit in the grooves and return holes of the oil guide pipe, which prevents the helicopter from achieving real-time alarm monitoring. The gearbox structure, where the metal particles are easy to deposit, should be improved.