Abstract: Pressure waves generated by the underground train passing through a tunnel can cause bad effects on the safety of the train operation and the comfort of passengers. To understand the characteristics of pressure fluctuation outside underground trains passing through a tunnel, the three-dimensional unsteady compressible Navier-Stokes equations are employed in this study to simulate this dynamic process, and the influence on the pressure fluctuation amplitude outside the train by various factors, such as the train speed, blockage ratio of the tunnel, length of the train formation form, length and hood structures of the tunnel, is quantified. The results show that, the pressure fluctuation amplitude outside the train increases with the 2.6～2.9 power of the train speed. As the blockage ratio of the tunnel is increased, the pressure fluctuation amplitude is also increased, but such an effect by the cross-section shape of the tunnel is weak for a fixed cross-section area. The lengths of the train formation form and the tunnel can cause a non-monotonic distribution of pressure waves along the train, thus an optimization design is required targeting the pressure fluctuation level at certain locations of the train. It is noticed that the tunnel hood can suppress the pressure fluctuation outside the train. Overall, the present study can provide guidance for the airtight design of the high-speed subway.