Underwater oil tree is the key equipment for marine water and oil development. Due to the long-term blockade of the design and manufacturing technology of offshore oil and gas exploration equipment in the United States,the design,manufacture and testing of marine trees depend entirely on foreign countries. The complex structure of the tree causes a large degree of welding restraint and a large increase in welding stress. The welding structure,the process,and the design of the welding groove will affect the welding quality. The hardness of welded joint of underwater oil tree has always been the focus of attention.In this paper,the hardness changes of the used materials sa-516 gr.70,GMAW welding conditions,different line energy and different welding joints of the welding joints are studied. The optimum welding parameters and heat treatment temperature were found by experiment.