Abstract: Homogeneous specimens of the laser-arc hybrid welding heated affected zone (HAZ) of low alloy high strength steel were prepared by welding thermal simulation technology. The instrumented impact test and microstructure characterization technologies were used to analysis the relationship between the microstructure and toughness of the simulated specimens. The results showed that the simulated coarse grained HAZ (CGHAZ) and fine grained HAZ (FGHAZ) composed of lath martensite (LM) and the inter-critical HAZ (ICHAZ) compose of LM and grain boundary carbide, the sub-critical HAZ (SCHAZ) is comprised of tempered martensite. The peak temperature has little effect on the crack initiation energy, but large effect on the crack propagation energy. The simulated ICHAZ and CGHAZ specimens have poor resistance to crack propagation. When the peak temperature is the same, the impact energy of the simulated CGHAZ specimens have little change with various the cooling rates. The peak temperature mainly affects the crack stable propagation energy and the crack stable propagation energy decrease as the peak temperature increase. The fracture process of the simulated CGHAZ specimen was controlled by the crack propagation, and the block was the microstructure unit controlling the crack stable propagation.