Abstract: The impact absorbed energy of various regions of the heat affected zone(HAZ) of submerged arc welding actual welded joint for 09MnNiDR low temperature pressure vessel steel was measured, the microstructures, fracture surface were observed and anlyzed. Based on the experimental results, the microstructures of all regions of HAZ, the weakest region can be determined, and the effects of the weakest region on impact toughness of the welded joint were further discussed. The results show that at −70 °C, the average impact absorbed energy of the base metal, subcritical heat affected zone, intercritical heat affected zone, and fine grain heat affected zone of the welded joint is above 270 J, which all show good toughness. The average impact absorbed energy of the weld metal is 139 J. The coarse grain heat affected zone (CGHAZ) is the weakest region of the welded joint. When the notch tip is completely composed of CGHAZ, the impact absorbed energy of 20 J is reached. The impact energy of CGHAZ decrease to 92.7% of the base metal. The microstructure of CGHAZ is composed of coarse granular bainite, lath bainite and block ferrite. With the increase of the fraction of CGHAZ in front of the notch tip, the closer from notch location is, the lower the impact absorbed energy of the specimen is, which fully reflects the influence of the weakest region on the impact toughness.