Abstract: Because cold transverse cracks occur usually in repair welding for large-scale thick plate of high-strength steel,in order to investigate the induced cause of transverse crack in repair welding,a new experimental method to produce delay transverse crack,both-end restraint crack experiment by small window,is developed for high strength steel.The results of microstructure investigation and hardness measurement show that the large-scale transverse cracks are found in repair welding zone,the angle between plane of crack and welding direction is 45°,and this macro crack is a hydrogen-induced crack because of high hydrogen contents and high restraint stresses.Lots of micro transverse cracks are found in root fusion zone of repair welding,the angle between plane of crack and welding direction is 90°,and this micro crack is a cold crack due to hardened structure.