Based on the Lagrangian approximation method

the motion equation of the displacement scheme considering the liquid-structure interaction is established

and a three-dimensional（3-D）finite element model of the vertical tank is constructed by using ANSYS.There natural seismic waves with rich short-period content（S1）

rich moderate-period content（S2）and rich long-period content（S3）are selected as inputs

and the effect of seismic wave spectrum characteristics on the simulated results of liquid sloshing

shear force and bending moment of the base

and tank wall stresses are analyzed.The results show that there are two obvious frequency bands in the natural frequency of the tank-liquid system

i.e.

the low frequency band（long period）of liquid convective motion

and the high frequency（short period）of coupling vibration of the tankliquid system.The predominant period of the S3 seismic wave is closest to that of the liquid convective motion

therefore induces obvious liquid sloshing

which is 4 times higher than that under the other two seismic waves.The effect of tank-liquid coupling vibration on the axial stress response is the greatest.The predominant period of the S1 seismic wave is closest to the tank-coupled vibration period

therefore induces the largest axial compressive stress of the tank

which has exceeded the allowable stress and results in significant "elephant-foot" buckling deformation.The shear force and bending moment of the tank base are most vulnerable to the influence of the tank-liquid coupling vibration

and the effect of liquid sloshing is very small.The seismic excitation only causes a convective motion of small amount of elements near the liquid surface.It is concluded that the dynamic behaviour of the fluid-tank system is highly sensitive to frequency characteristics of the earthquake record.