Abstract: By considering the change of δ_CTOD value and residual stress after post weld heat treatment（PWHT）, the effects of PWHT on the complex structure integrity were investigated. The residual stress and fracture toughness (CTOD) of 80 mm thick plate butt joints in as-welded state and after heat treatment were tested. It is found that heat treatment could effectively reduce the residual stress of thick plate butt joints, and the δ_CTOD of weld metal kept at the same level while the HAZ decreased substantially. The residual stress of welded structure with high constraint and assembly stress is measured in as-welded and heat-treated states. The results show that the local heat treatment could not effectively reduce the residual stress of complex structure, and even rise instead in some parts. Therefore, combined with structural integrity assessment, the study results demonstrate that local heat treatment was not able to improve the safety of complex structure.