The research on structural design parameters of 400 km/h highspeed railway tunnel is being carried out in order to promote the technical standards of highspeed railway tunnels in China, and in which the tunnel clearance area is one of the key items. The method for determining the clearance area of highspeed railway tunnel on the basis of passenger ear comfort standard is established from the aspect of rail vehicle internal transient pressure. Based on the control condition, the probability for maintaining existing tunnel clearance area is discussed in terms of rail vehicle airtightness by calculation and analysis. And the suggested clearance area of 400 km/h tunnel is proposed. The conclusions are drawn as follows: (1) The lowest train dynamic airtightness index is 22 s when the existing 350 km/h singleline tunnel operates at a speed of 400 km/h, and the rail vehicle internal transient pressure exceeds the passenger ear comfort standard may be a high probability event. (2) The suggested clearance areas of singleline and doubleline 400 km/h tunnels are 85 m2 and 100 m2 respectively, and the corresponding train dynamic seal index is 18 s, which is more reasonable to the existing train and therefor the risk of exceeding standard can be greatly reduced. (3) The suggested clearance area for 400 km/h highspeed railway tunnels and corresponding airtightness requirements can be taken as the reference for relevant standards and specifications.