The stress relaxation phenomenon of bolt used in thermal recovery wellhead is studied experimentally under field conditions.The normal and high temperature tensile test,creep test and stress relaxation test are carried out respectively.The comparison of the experimental results shows that the strength of bolt material decreases and the plasticity increases at high temperature.The creep deformation is obvious under initial stress which explain the reason of stress relaxation in principle.The stress relaxation test,it can be found that the bolt stress have decreased obviously under the field condition.According to the stress relaxation experimental data,the stress relaxation formula under the field condition can be fitted.Finite element software is used to calculate the minimum pre-tightening load of bolts for each flange of KR21-370 thermal recovery wellhead device to achieve sealing.Combined with stress relaxation formula,the time required for flanges to fail due to stress relaxation of bolts is calculated.Solutions are proposed for bolts prone to failure.