Chaohe tunnel on Central Route of SouthtoNorth Water Transfer Project consists of twin parallel circular tubes. The tunnel is 18.147km long. The clear spacing between the twin tubes is 26.6m. The designed diameter of the twin tubes is 12.1m. The twin tubes are to be bored by 4 EPB shield machines from the tunnel portals. Due to the long distance of the tunnel, it is difficult to execute construction ventilation. The required air flow and air pressure are calculated and the required ventilation methods and ventilation parameters are determined on basis of the tunnel layouts, construction methods, major pollution sources and health standards of the working environment in the tunnel.