During the actual operation of a high-speed train, the airtight performance of the carriages will change significantly. Based on a series of field tests, the external and interior pressure variations of train carriages are measured and then the dynamic pressure tightness coefficient are analysed quantitatively for investigating the airtight performance of a train passing at various speeds through tunnels of different lengths. It is found that the amplitude of the external pressure variations is proportional to the square of the train speed, while the amplitude inside the vehicle is approximately proportional to the speed to the 0.7th power due to the airtight effect. The dynamic pressure tightness coefficient increases with increasing tunnel length and decreases with increasing train speed. Regarding the effect of the carriage location, the coefficient shows an overall increasing trend from the train head to tail.