Abstract: The different pressure fluctuation caused by a high-speed train passing through tunnels of various length can cause different degrees of pressure comfort problems for passengers. The one-dimensional compressible unsteady non-isentropic flow model characteristic line method and the time constant method pressure tightness index model were used to study the pressure wave outside the train and the pressure change characteristics inside the train under two pressure tightness indexes when a single high-speed maglev train passes through the tunnel. The concept of the critical tunnel length of the high-speed maglev single line based on the pressure comfort standard was improved, and the influence of the train speed and train dynamic pressure tightness index on the critical tunnel length was studied. It is found that: under the condition of critical tunnel length based on the maximum negative pressure value of the external pressure, the maximum negative pressure value of the internal pressure is smaller. The maximum value of the maximum pressure change in each 1, 3, 10 and 60 s in the train increases first and then decreases with the increase of the tunnel length, and there is the critical tunnel length under pressure comfort constraints. The critical tunnel length at different train speeds is different. Except for per 10 s limit conditions, the critical tunnel length under different train dynamic pressure tightness indexes is approximately the same. When a 600 km/h single-train maglev train with a dynamic pressure tightness index of 83 s passes through a 100 m² tunnel, the critical tunnel length based on the UIC660 comfort standard is 10–12 km. The research results of this paper have good reference value for the study of tunnel clearance area and train pressure tightness based on comfort standard, and for further improvement of the theoretical system of the critical tunnel length of the rail transit based on the tunnel pressure wave effect.