Abstract: High heat input will aggravate grain coarsening in the heat-affected zone (HAZ) in the process of large heat input welding, which easily leads to the decrease of impact toughness in HAZ. In this paper, the 90 mm thick S460 steel plate was taken as the research object. After heat treatment at different temperatures ranging from 400 ℃ to 1 400 ℃, the low temperature impact test at -20 ℃ was carried out. The metallographic structure, fracture macroscopic and microscopic morphology were characterized by means of metallographic microscope, superfield three-dimensional microscope and scanning electron microscope. The results showed that between 400 ℃ and 750 ℃, with the increase of temperature, the original ferrite grains in the metallographic structure gradually grew up and the content decreased, and the pearlite formed hard phase M-A island component after austenitizing and cooling. In the range of 800～1 400 ℃, the content of ferrite was basically unchanged and the size of granular bainite groups increased. The impact absorbing energy of -20 ℃ decreased with the increase of heat treatment temperature,especially in the range of 650～750 ℃, the impact absorbing energy decreased sharply. The results of macro and micro analysis of impact fracture were consistent with the law of impact absorbing energy.