The analysis of pressure waves induced by passage of highspeed trains in a tunnel is a key in the design and construction of tunnel, which is one of the main factors for determining the crosssection of tunnel.  Based on many investigations in some countries, the onedimensional compressible unsteady flow model and the  characteristics method  are presented in detail in this paper.  The model can be used to calculate the pressure waves induced by passage of a train or two trains in a tunnel without or with shafts.