Abstract: In this study,the submerged arc auto-welding technique was employed to weld the high strength steel with low welding crack suceptibility.The relationship between microstructural feature and toughness in different sub-zones of heat affected zone was analyzed in details.The results showed that the main microstructures in the coarse grained zone and fine grained zone are coarse bainite and fine quasi-polygonal ferrite,respectively.And corresponding average impact absorbed energy is 45 and 170 J,respectively.Electron back scattering diffraction(EBSD) analysis showed that the effective grain boundary of coarse bainite is prior austenite grain boundary and a number of sub-boundaries and dislocation exist inside the grain.The coarse effective grain size reduces the toughness of coarse grained zone.On the contrary,the average effective grain size of quasi-polygonal ferrite in the fine grained zone is about 5.3 μm.High angle grain boundary inhibits the propagation of crack and improves the toughness.