The prediction of the pressure wave amplitude produced when two trains pass each other in the tunnel is important to the train design for airtightness and tunnel conditions in China. In this paper, the key factors of this problem were firstly studied based on theoretical analysis. The equation of the worst tunnel length for the global maximum and minimum pressure values was derived. Then, the influence of tunnel length on global minimum pressure and the critical region in which the global minimum pressure varies rapidly were investigated. Finally, a numerical method based on two-dimensional Navier-Stokes equations was established. Typical conditions of two trains passing-by in tunnels of different lengths were simulated. The theoretical and computational results agree with each other closely.