An axisymmetric finite element model of thermal-structural mechanics is established based on the actual drilling data in the east of the South China Sea and the theory of thermodynamics and structural mechanics to calculate the axial thermal expansion force at the wellhead above seabed by ANSYS software. The axial force caused by thermal expansion of the casing string is calculated by classical theory, so as to evaluate the sealing integrity of the Annulus Sealing Assembly (ASA) in the long term. It is indicated that the axial upward force caused by fluid heat circulation at the casing hang is 2 811 kN under the condition of 4 285 m well depth and normal temperature and pressure, but the working capacity of the ASA lock ring is 3 247 kN, so the ASA lock ring can resist the upward force due to thermal expansion. In order to guarantee the sealing integrity of the ASA throughout the design life of the well, it is recommended to use a lockdown device at the top of the casing hanger to prevent the vertical movement of the casing hanger and ASA. The methods and measures are of useful reference for the integrity evaluation of the wellhead annulus seal during production.