Abstract: An explosion inside the tunnels would generate blast wave which transmits through the longitudinal tunnels. Because of the close-in effects of the explosion and the reflection by the confining tunnel structure, blast wave propagation inside the tunnels is distinct from which in air. When the explosion happens inside tunnel structure, the peak overpressure is higher than which the explosion happens in air. The duration of the blast wave also becomes longer. With the help of the numerical simulation finite element software LS-DYNA, a three-dimensional nonlinear dynamic simulation analysis for a experiment of the explosion inside the tunnels has been made. LS-DYNA is a fully integrated analysis program specifically designed for nonlinear dynamics and large strains problems. Compared with the experiment results, the simulation results have made the material parameters of numerical simulation model available. Making use of the model and the same material parameters, many results have been adopted by calculating the model under variety for different TNT explosion dynamites .Then the method of dimensional analysis has been used for the simulation results. As the explosion blast wave overpressures are the governing factor in the tunnels response, a formula has been suggested for calculating the blast wave peak overpressures at a certain distance from the detonation center point inside the tunnels by using the dimensional analysis theory. Compared the results computed by the formula with experiment results which had been done before board and in, the formula has been proved very applicable at some instance. The research may be helpful to rapidly estimate the tunnel internal explosion effects on the structure.