Given the poor accessibility of the current finishing process for parts with complicated geometries, a novel bidirectional composite vibratory finishing (BCVF) approach is presented, which combines the power actions on abrasive particles and processed workpieces. The discrete element method (DEM) was used to obtain a fundamental understanding of the BCVF process. By comparing different processes, it has been demonstrated that the BCVF promotes the mobility of abrasive particles to improve the finishing results and efficiency further. The characteristics of normal and tangential contact on the surface of the workpiece involved in the BCVF process were analyzed. Meanwhile, the effects of container size (or wall effects), media amount, workpiece position, and vibration parameters (including vibration amplitude and frequency) on the media-component interactions were systematically studied. The results show that the distance between the workpiece and the container wall in the y-direction can be reduced to 4d (d is the abrasive particle diameter) without affecting the finishing effect. In addition, with the enhancing vibration in the x-direction, particle impact and shear effects are subsequently strengthened. In contrast, the media amount above the workpiece and the z-directional vibration is mainly effective for the shear effect. Overall, the process can enhance the impact and shear effects simultaneously through composite vibration.