A numerical model based on computational fluid dynamic software FLUENT is established to simulate the air distribution and purifying effect under different air pressures in both the tunnel and the bypasstype purification station. It is shown that: 1) The air flow rate of bypass tunnel only depends on the resistance of equipments and fan performance rather than the condition of main tunnel. 2) When the upstream air flow rate is less than that of the station, “backflow” phenomenon occurs and results in unbalanced purifying load of the two bypass tunnels. 3) When the air flow rate of the station to the main tunnel is 100%, the purifying effect is the best. The study results can provide reference for design and operation of ventilation and purification system of underground works in the future.