Abstract: Homogeneous specimens of the double-pass laser-arc hybrid welding heat affected zone (HAZ) of low alloy high strength steels were prepared by welding simulation technology, the influence of second peak temperature on the microstructure and toughness of the simulated specimens was investigated. The results showed that the unaltered coarse grained HAZ (UACGHAZ) compose of coarse lath martensite (LM) with an average grain size between 84 − 98 μm. The supercritically reheated CGHAZ (SCRCGHAZ) is comprised of fine LM with an average grain size between 15.7 − 19.2 μm. The intercritically reheated CGHAZ (ICCGHAZ) compose of LM with blocky martensite-austenite constituents distributed along grain boundaries and subgrain boundaries. The subcritically reheated CGHAZ (SRCGHAZ) is comprised of LM with an average grain size between 79 − 88 μm. The instrumented impact test results showed that the ICCGHAZ specimens own the lowest resistance to crack initiation, and the ICCGHAZ and UACGHAZ had the poorest resistance to crack propagation.