Pressure transient is generated when a high-speed train passes through a tunnel, while the pressure inside the coaches will also fluctuate. It is one of the main factors affect the aural discomfort of passengers. The train speed, blockage ratio, and the length of the tunnel also affect the pressure transient inside and outside the coaches. The characteristics of pressure fluctuation in super long tunnels are different from that in middle-length tunnels. The applicability of the criterion in which the maximum pressure change in a single time interval is used to evaluating the aural discomfort in China is investigated. And it’s necessary to study the characteristics of pressure fluctuation. Based on one-dimensional, compressible, non-homentropic and unsteady flow model and the method of characteristics of generalized Riemann variables, characteristics of pressure fluctuation in a plain tunnel are studied. The effects of tunnel length, train speed and pressure tightness coefficient on the pressure fluctuation inside the coaches are studied. After analyzing the variation of the maximum value of maximum pressure change during different time intervals, the particularity of pressure fluctuation inside the coaches when the train passes through the super long tunnel is obtained. Based on the pressure comfort criterion in European, the characteristic of aural discomfort are summered. And the conclusion is that it’s necessary to estimate the aural discomfort through the pressure changes in different time intervals in super long tunnels.