Abstract: Fracture toughness test and scanning electron microscopy(SEM) were adopted to investigate the effects of post-weld heat treatment(PWHT) on the joint weld, coarsened grain zone, and CTOD properties of line between subcritical HAZ and intercritical HAZ of welded joint of S420 ML high-strength steel for offshore engineering.Resultsshowed that the CTOD properties of 80 mm thick S420 ML welded steel plate could satisfy the requirements of relevant standards whether the plate went through post-weld heat treatment or not, and that the CTOD properties of welded plates were significantly improved after PWHT at 580℃ for 4 hours. The crack propagation surface of specimen with characteristic value of δ_m were mainly characterized by fiber zone, and for the specimen with characteristic value of δ_u, the initial stage of the crack propagation surface had some brittle fracture regions, then due to the release of load, the crack propagation stopped, and the cracking stopped in time. The crack propagation resistance was regained during subsequent loading, so that the remaining part of the fracture still presented the characteristic of fiber fracture.