Abstract: Based on the analysis of Zr alloy weldability, the welding test of 30 mm thick 705C Zr alloy plate was carried out by using manual gas tungsten arc welding in vacuum box.The microstructure and mechanical properties of the welded joint were analyzed by optical microscope observation, tensile test, bending test and hardness test.The results show that by increasing the welding current properly, controlling the vacuum degree and interlayer temperature, the welded joint of Zr alloy thick plate with high quality can be obtained.The microstructure of the weld zone is fine, dense and small strip α phase, the microstructure of heat affected zone is not coarsened seriously, and the α phase is a little coarser than that of the weld zone.The tensile strength, elongation of the joint reaches more than 97%, 90%of the base metal, respectively, the bending property is qualified, and the hardness distribution of the cross section of the joint shows no brittness and hardening phenomenon.