Considering that seismic soil pressure is the key to an efficient seismic design of an underground tunnel, seismic soil pressure on an underground tunnel in the axial direction is studied by indirect boundary element method. First, the time domain problem is transformed to discrete frequency domain problem by Fourier transform. Next, the free-field ground motion is calculated, and the scattered field is simulated by a set of fictitious loads combined with the Green' s functions of uniformly distributed load acting on an inclined line. The values of these fictitious loads are determined from the displacement boundary conditions of the underground tunnel. Then, the seismic soil pressure in the axial direction is obtained by utilizing these fictitious loads. Finally, the seismic soil pressure is transformed back to time domain by inverse Fourier transform. The proposed method is verified by comparing its results with those of the analytical solution. Based on the model of a rigid underground tunnel soil layers over elastic bedrock, the amplitude and distribution of the seismic soil pressure are analyzed through parametric studies. Several findings are obtained. (1) For the underground tunnel, the kinematic interaction plays a major role while the inertial interaction has little effect. (2) The dynamic soil-tunnel interaction (SSI) only has minimal effect on the distribution of the seismic soil pressure, but it can significantly affect the amplitude of the seismic soil pressure; moreover, the peak of the seismic soil pressure can be significantly amplified compared with the relevant free-field soil stress. (3) The distribution of the seismic soil pressure is similar with that of the free-field soil stress, which is nearly linear distribution. Furthermore, the seismic soil pressures on the top and the bottom of the underground tunnel are in opposite direction, whereas the seismic soil pressure on the bottom of the underground tunnel is large. (4) Finally, the burial depth of the underground tunnel affects the amplitude and the distribution of the seismic soil pressure. Furthermore, the amplitude of the seismic soil pressure increases with the augmentation of the burial depth. A simplified estimation method is proposed for analyzing seismic soil pressure in accordance with its basic law. The simplified method is based on free-field seismic response analysis, which does not require the calculation of the dynamic soil-tunnel interaction. The method can be utilized to estimate the design load for underground tunnel seismic design in the axial direction.