Abstract: Objective To provide passengers with a comfortable, healthy, and safe travel environment, it is necessary to analyze the changes in air quality inside rail transit vehicles during the tunnel operation. Method Based on the regulations in Chinese and international standards regarding the fresh air volume requirements for rail transit vehicle air-conditioning systems, the Beijing-Zhangjiakou High-speed Railway tunnels are taken as example, considering factors such as the permissible mass fraction of in-car CO₂ and the influence of the in-car pressure wave damper control logic, the measured data of CO₂ mass fraction variation in an EMU（electric multiple unit） tail car is used to calculate and analyze the CO₂ mass fraction variation in the intermediate car under different operating conditions. Result ＆ Conclusion The in-car CO₂ mass fraction variation is related to the opening/closing of the pressure wave damper, different fresh air volumes, vehicle airtightness, and train tunnel operation time. Due to the inherent leakage in the vehicle, the actual fresh air intake is higher than the theoretical value. If the fresh air pressure wave damper is closed when the train enters the tunnel, the time taken for the in-car CO₂ mass fraction reaching 0.250 0% under operating conditions one, two, and three is 13 minutes, 9 minutes, and 7 minutes, respectively; the time taken for the in-car CO₂ mass fraction reaching 0.500 0% after train entering tunnel is 36 minutes, 32 minutes, and 26 minutes, respectively. Given the long travel time within extended continuous tunnels, it is recommended to control the in-car CO₂ mass fraction within 0.500 0%, and to appropriately increase the interval time for opening the pressure wave damper based on actual line conditions to ensure pressure comfort.