The performance of weathering steel for fabricated building was studied by thermal stimulation technology. The results show that coarse-grain heat affect zone（CGHAZ）,which gets the highest hardness and the poorest impact toughness,is most possible to became the weak areas of welded joint. With increasing of heat input,the microstructure of CGHAZ changes from lath bainite to granular bainite,and the hardness decreases. The impact energy increases at the beginning and then decreases. The maximum impact energy reaches 236 J when heat input is 15 kJ/cm.The fracture behavior of those specimens are further analyzed by observing fracture morphology and section microstructure of those impact specimens. When heat input is too low,more quenchinghardening structure,which get little small plastic deformation during fracture process,is formed,and impact toughness is poor.When heat input was over 20 kJ/cm,the ability of preventing crack propagationdecreasesandimpacttoughnessdecreases.