Based on the unsteady compressible three-dimensional N-S equations and standard k-turbulent model, the travel process was simulated used the finite volume method. The pressure variation mechanism and its spatial-time change characteristic of PSD were analyzed. Meanwhile, the relationship between the pressure of PSD and such influence factors was discussed, as train speed, blockage ratio, the area as well as the ventilation scheme of the subway tunnel. Result showed it is necessary to consider the pressure variation in influence of the exhaust ventilation of the subway tunnel, and the pressure gradient along with the train and the pressure wave between the piston vents, coming into being during the train traveling in subway tunnel, are the basic factors causing the pressure fluctuation of PSD, especially the former, which is called the plane turbulent Coquette flow. The scope of PSD pressure fluctuation is proportional to the tunnel blockage ratio and the square of train speed. Smaller area of piston vent is favorable to reduce the pressure.