To investigate the influence of damaged duct on carbon monoxide's emission during tunnel construction, relying on the Ji-ming Tunnel of Cheng-kai Expressway. The orifice flow theory was used to analyze, and the velocity in duct, the distance between holes and working face and the number of holes were considered as factors. The orthogonal test was carried out to research the influence of duct damage on the distribution of CO. The results show: the air leakage rate would be increased by the increase of the air of the working face. When the ventilation time is 1min, the CO concentration would research its peak at the working face, which is about 3.5%. A lag effect on the dilution of CO in the tunnel would be caused by the damage of duct. And the lag time would be 1 to 3 minutes, and it would be more obvious with the decreasing of air demand at working face. The result that the concentration of CO within 20m from the entrance of the tunnel being higher than the requirements finally was also caused by the damage of duct. In ventilation region, the influence degree of the three factors on the dilution and discharge of CO is: the velocity in the duct > the distance between the holes and working face > the number of holes; In the whole tunnel, the order turns into: the velocity in the duct > the distance between the holes and working face> the number of holes. There are suggestions on ventilation design and holes repair.