In order to explore the diffusion and pollution mechanisms of high-concentration dusts produced in the fully-mechanized excavation face towards the operation area in the tunnel at mesoscopic scale, this article investigated the diffusion behaviors of dust particles based on CFD-DEM coupling model and field measurement. The results show that the wind vortex fields, specifically, two large horizontal vortex fields (2-10 m and 10-25 m away from the heading face) and a large vertical vortex field (0-5 m away from the heading face). For dust particles of various sizes, the diffusion distance exhibited a linearly increasing trend versus time while the maximum diffusion velocity followed a logarithmically decreasing trend with respect to the diffusion distance, the ratio of trajectory deviation of the dust particle exhibited an nonlinearly increasing trend versus dust particles sizes. The settlement of dust particles mainly occurred in two regions, which were 0-8 m and 25-35 m from the heading face. Moreover, larger dust particles settled more easily, with the settling rate up to 84.5%, while fewer respirable dusts settled, only approximately 46.7%. The dust prevention should be adopted by considering the specific diffusion distance and pollution mechanisms of the dust particles with different sizes. (C) 2016 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.