The soft interlayer under seafloor has great influence on the stability of subsea casing string for deepwater production. Especially

the earthquakes can cause a great bending moment and large shear force on the cross-section of casing string. An analytical model for the seismic response of subsea casing string is established on the basis of pile foundation theories and method of reverberation-ray matrix. The casing string is assumed to be a Timoshenko beam of variable cross-section. The bending moments and shear forces of different casing sections are discussed from the aspects of the shear wave velocity and thickness of soft interlayer. The result shows that while shear wave velocity of soft interlayer decreases or its thickness tends to the critical value hcr

the bending moment and shear force of casing string at interfaces of soft interlayer will increase accordingly. Therefore

influence of soft interlayer on seismic response of casing string should be fully taken into account in marine engineering.