Abstract: In order to obtain the dynamic characteristics of tunnel in loess surrounding rock under seismic action after moistening， the seismic dynamics of the loess surrounding rock with moistening and tunnel structure had been analyzed with the time-history method， basing on the relationship between initial elastic modulus， the shear strength parameters and the water content. The numerical computation results showed that， with the water content of the loess surrounding rock increasing， the absolute value of principal stress and the autooscillation circular frequency all decreased relatively， while the period of free vibration increased correspondingly. When the water content was less than its critical value， the peak acceleration of the tunnel roof was larger than the original seismic peak acceleration. Otherwise， it was opposite. Under the same water content， the maximum and minimum principal stress appeared alterna-tely， making the tunnel lining material fatigue damage， which was the main reason for the tunnel lining damage. The results can provide a reference for the seismic analysis on the different water content conditions of loess surrounding rock and tunnel structure.