Abstract: Hot cracking has become one of the main problems in GTA welding of thick copper plates. To find the dominant factors influencing the hot cracking, the hot cracking tendency of thick copper plate GTA welding with HS201 wire was investigated. The sensitivity was evaluated by the rigid restraint cracking test in air and an argon atmosphere. Combined with the test results of the high temperature ductility of the HS201 filler metal and deposited metal. The main cause and the corresponding inhibition measure of the hot cracks were revealed. Results indicate that, the main cause of the hot cracks were due to the poor deoxidizing capacity of the HS201 wire. Oxygen invaded the weld pool, forming low melting eutectic of Cu₂O and Cu. The eutectic segregated at the crystalline columnar crystal boundary, which cracked under stress.