Because of remarkable nonlinear behavior and solid-liquid two-phase character of the soil under seismic loads

the soil dynamic properties will be changed significantly. The dynamic intensity and the deformation of soil are mostly depended on the development of pore water pressure. Both of them will influence the dynamic response of the soil-pipeline system. Therefore

the increase of vibrant pore water pressure and the reduction of effective stress should be taken into acount for their effects on soil dynamic properties. On the other hand

the coupling between the dissipation and the re-distribution of pore water pressure and the soil frame response should also be considered. In this paper

the soil surrounding the pipeline is regarded as a solid-liquid two-phase medium. The effective stress method and nonlinear constitutive relation model of soil are used to study the development and the dissipation of pore water pressure during a seismic process. At the same time

the contact interface model is adopted to simulate the interaction between the pipeline and the surrounding soil. Using the program SRUPFE based on the theory of this paper

a numerical simulation of the full-process analysis for an underground pipeline can be gained accurately. From the results of calculations

the development of the pore water pressure leads to the soil softening which causes the change of the soil dynamical characters

and the spectrum identities of soil-pipeline system are also changed. Under horizontal seismic loads

for the stress of pipeline

the axial stress is much more than the flexural stress. Moreover

the seismic response of soil-pipeline system changes with the variation of the soil softening level.