The influence of nonuniform distribution of air velocity and contaminant concentration on purifying effect in highway tunnel can not be analyzed by the existing theories. In order to accurately predict the pollutant distribution in the whole tunnel with purification station, a threedimensional numerical model which includes the coupled effects of jet fans, moving cars and purification station is established and validated.The model can predict the air flow and pollutant distribution under different ventilation, purification and traffic conditions; and it is used to simulate an actual urban tunnel with bypasstype purification station.The results show that the air flow in the tunnel is not onedimensional, and air circulationis found in the main tunnel between the inlet and outlet of the purification station; the purification station can decrease the air flowrate in the main tunnel adjacent to the purification station, which results in the maximum concentration of pollutants in the main tunnel close to the outlet of the purification station.It is therefore suggested that this recirculation phenomenon should be taken into consideration and the pollutant concentration in the main tunnel close to the outlet of the purification station should be taken as the check point when designing such kind of ventilation and purification system of highway tunnel.