Abstract: Lots of harmful gas release during tunnel construction by D&B method, and the ventilation and health protection during the tunnelling process is of great significance. Taking the Tianmu Mountain tunnel on the Hangzhou-Huangshan high-speed railway as the background, real-time monitoring of air quality after blasting was conducted and concentration variations of harmful gas before and after ventilation were compared to analyze the timespatial distribution law of the components of harmful gas and evaluate the ventilation effect. The results show that the air quality meets the standards after ventilation for 15 min with a volume of 2 688 m3/min required by dilution of exhaust gas emitted by internal-combustion engines, and the harmful gases do not exceed the standards for all the working shifts, which proves good ventilation effect. The results provide significant guidance for the environmental air quality control and health protection during tunnelling.