Abstract: Take tunnel Doupengshan as the research background, in order to study the evolution characteristics and diffusion rule of free SiO₂ dust concentration in the double heading tunnel, and to obtain reasonable designing parameters of ventilation system, the diffusion path and concentration distribution of dust were simulated in different time periods before and after dust removal equipment was installed by the discrete term model of Ansys-CFD (a FEM computational fluid dynamics software). Meanwhile, the simulation results were compared with the field measured data. The results indicate that:(1) Lining platform car had a certain obstructive effect on free SiO₂ dust, and there was a clear demarcation of the concentration of free SiO₂ dust on both sides of platform car. (2) Some high concentration free SiO₂ stayed for a relatively long time in non-blast zone, small amount of high concentration free SiO₂ passed through the platform car to form cluster effects in non-blast zone after a period of ventilation and was undischarged by shaft just under forced ventilation. (3) The concentration of free SiO₂ dust approximated to 0 in non-blast zone in the joint operation of forced ventilation and dust removal system, and the concentration of free SiO₂ dust was reduced to less than 8 mg/m³ in the main tunnel and shaft after 900 s of the joint operation, which met the regulatory standard. The research result shows that:it was feasible to simulate the evolution characteristics and diffusion rule of free SiO₂ dust concentration in the double heading tunnel by the discrete term model. Under the joint operation of forced ventilation and dust removal system the free SiO₂ dust entered the shaft for discharge quickly and the concentration of free SiO₂ dust decreased sharply in construction area within a short period of time, and the diffusion path of dust almost did not reach the non-blast zone which reduces pollution to non-blast zone in tunnel.