While the gear engagement working, debris particles are generated and theoretically brought away by flowing lubricant oil. The conveying process of particles in lubricant is helpful to clean up the friction pair, but how are the particles brought out and how does the particles convey in oil are still unknown. Electrostatic sensor can be used to monitor the debris particles in lubricant while the friction pairs are working. If the debris distribution in lubricant can be known before the installation of the sensor, it may get a chance to position the sensor on the way where particle concentration is the highest. The research result can be used to guide the installation of electrostatic sensor and increase the output efficiency of the sensor. In this paper, the CFD method is used to study the conveying process of particles, combined with the Dynamic Mesh technology and the DPM model, and an external gear box is taken as an example for analysis. Different sizes of particles are put into the lubricant oil flow field, and the trajectories and distribution of particles are calculated by simulation. The calculated result found that The movement of the particles in the gear box is very complicated, with a large randomness. All the particles show a tendency of centrifugal movement, most of the particles are distributed close to the inner wall of the box and less far away from inner wall of the box. When the particles are flushed out from the gear box, they are more distributed in the lower half of the pipe. The small particles have a better followability in oil flow and are easy to be affected by turbulence. And when they are discharged from gear box into the pipeline, the average velocity of them are larger. Whereas the large particles are in contrary. In addition, the faster the gear's working speed and the smaller the centre distance (engagement gap) are, the more significant the particle is affected by turbulence, and the particle trajectory shows greater cross-disorder and randomness. The average velocity of particles motion has a nonlinear positive and negative relationship with gear speed and centre distance.