Abstract: The airflow field and contamination migration during the construction ventilation of the fourth layer of underground powerhouse is simulated taking into account influences of the branch tunnel and the main transformer chamber on the ventilation of the underground powerhouse. The velocity decreases along with increasing distance from the heading face. And the velocity at a majority of underground powerhouse is less than 0.15m/s. There is an eddy zone at the X-Y section and a high concentration range of the contaminant around the outlet. The migration rule of CO is similar to the migration tendency of the dust. A comparison between the concentration field predicted by the model and the experimental data of Nakayama et al. indicates a good agreement.