Abstract: The formation mechanism of hot cracking in gas tungsten arc welding(GTAW) of copper structures in large dimensions was researched. The dynamic formation process of hot cracking was observed and analyzed. The formation criterion of hot cracking was optimized based on the Prokhorov's theory, and the finite element model of thick copper plates in GTA welding was established based on the rigid restraint cracking test. It is concluded that the internal deformation rate Δε is the internal reason of forming hot cracking. The variation of the transverse tensile stress and the Δε in brittle temperature range(BTR) was obtained. And the formation mechanism of hot cracking without preheating was carried out compared Δε with high temperature ductility of HS201 welded metal. The variation of Δε in difference preheating temperature was analyzed to prevent hot cracking forming, and it can be concluded that the Δε in BTR could be declined by preheating process and the cracking susceptibility will be decreased.