Tunnel construction ventilation can effectively eliminate harmful gases and dust, and ensure the safety of construction environment. Taking an extra-long road tunnel as an example, the air demand for the construction of inclined shafts and tunnels was calculated with multiple working faces, choose the appropriate fan models, and the tunnel construction ventilation in stages was designed according to the construction progress of inclined shafts and main caves. A section of the split tunnel was used as a ventilation tunnel, and the roadway + pressurized ventilation method is adopted to shorten the length of the air supply ducts. FLUENT software was used to establish a three-dimensional numerical model of multi-working face construction ventilation, in-depth study of CO(carbon monoxide) diffusion regulations in tunnels. The results of the study show that CO migrates from near the tunnel face to the entrance in the form of “air mass”, and the migration rate is positively correlated with the ventilation wind speed in the tunnel.The location of the high CO concentration zone at the intersection of the inclined shaft gradually shifts to the right (low velocity zone) as the wind velocity ratio gradually becomes larger, and the maximum concentration gradually decreases.The wind speed ratio has no significant effect on the change of CO concentration in the left hole.