Transient pressure is one of the important factors affecting the structural safety of the lining of a high-speed railway tunnel. To further understand the characteristics of transient pressure and pressure gradient of a high-speed railway tunnel wall, a numerical simulation model for a high-speed train composed of eight cars with a scale of 1∶1 was established to investigate the aerodynamic characteristics induced by a high-speed train travelling through a double-track tunnel, based on the RNG k-ε two-equation turbulence model and the sliding mesh technique. The accuracy of the numerical method was verified by comparing the numerical simulation results with the field measurement. The influences of train speed, blockage ratio of train-tunnel, the number of trains, and two trains intersecting with the same speed on the transient pressure and pressure gradient on tunnel longitudinal middle section were further studied based on the analysis of the temporal-spatial traits of transient pressure on the tunnel wall. The results show that, the time-history curves of transient pressure in the tunnel cross-section are nearly overlapped, the pressure distribution in the cross-section is uniform, and the difference is low. Three typical pressure peak values (maximum positive peak value, negative peak value, and peak-to-peak value) show a linear relationship with the square of train speed or the square of blockage ratio, the positive peak value of the initial pressure gradient has a linear relationship with the third power of the train speed, the positive and negative peak values of pressure gradient have a linear relationship with the power of the blockage ratio (2.51~3.14). The relationship between the three typical pressure peaks and the number of trains satisfied an exponential function, the maximum positive peak value of pressure gradient of 8 trains is 1.25 times that of other cases (4~7 trains). The maximum positive and negative pressure peak values of the two trains intersecting with the same speed are 2.22 times and 1.15 times that of the single train passing by, respectively, and the maximum positive and negative peak values of pressure gradient of the two trains intersecting with the same speed are 1.87 times and 2.03 times that of the single train passing by, respectively. The relevant research results have important reference values for the safe operation of a high-speed train in tunnel.