When high-speed trains travel through tunnels, drastic pressure variations are generated. The pressure waves may cause aural discomfort and fatigue failure of the car body, which the basic problem is predicting the pressure wave rapidly. Computer programs which can predict the pressure wave caused by a single train or two crossing trains in the tunnel are introduced and validated. One-dimensional(1D), compressible, unsteady, and non-homentropic flow model and method of characteristics of generalized Riemann variables are used in these programs. Railway tunnel with typical length in Beijing-Shanghai high-speed rail line are studied. The generation of pressure wave is depicted through the spatial and temporal variations in pressure. The reason the pressure wave caused by two crossing trains is severer than that by the single train is qualitatively analyzed. And the effect of train speed and blockage ratio on the maximum and minimum external pressure excursion is studied. The result shows that the maximum pressure excursion is approximately proportional to the square of train speed and linear related to the blockage ratio when high-speed trains passing through or crossing in the tunnel of typical length in Beijing-Shanghai high-speed rail line.