This paper presents a Monte Carlo method for the seismic safety analysis of underground tunnels regarding the variation of design seismic excitations.（1）Establish the model for nonlinear seismic response analysis of underground tunnels

in which

the viscous-spring boundary method and equivalent earthquake node force input method are combined to represent the seismic excitation in half-space

and general finite element software ANSYS is utilized for the seismic safety analysis.This model uses the equivalent linearization method to describe soil nonlinearity

and combines Rayleigh damping and material damping to consider soil damping

and computes concrete and reinforcement nonlinearity separately

and also takes soil-tunnel contact into account.（2）Determine the design earthquake records

and generate some sample seismic excitations whose statistical characteristic is in accordance with the target evolutionary power spectral density function of the design earthquake records.（3）Make nonlinear seismic response analysis of underground tunnels under these sample seismic excitations

and compute the variation of the seismic responses.This study provides a simple and practical method for seismic safety analysis of underground tunnels

which is of certain value to seismic design of underground tunnels.