Abstract: The continuous cooling transformation curves(SH-CCT) in the heat affected zone(HAZ) of F550 steel plate were established with thermal simulation, and the microstructural evolution and mechanical properties of the HAZ were investigated.Experimental results showed that fine Zr and Ti complex oxides dispersed in the steel weld, which have pinning effect on the austenite grain in the coarse-grained heataffected zone(CGHAZ) due to the combined deoxidation of Zr and Ti in the steel plate.Meanwhile, the low carbon content and proper addition of Ni, Cr, Cu and Mo were adopted in the design of high-strength steel.Consequently, the bainitic microstructure with small amount of dispersing M-A islands was obtained in the HAZ in a large range of heat input(25-100 kJ/cm).The results of V-notch impact test of CGHAZ welded by submerged arc welding with heat input of 50 kJ/cm indicated that the impact toughness of F550 steel was good at -60℃.