BROCO underwater welding electrodes are used for multi-layer welding of 16 mm thick Q345 steel plates under onshore condition and underwater conditions,respectively. Microstructures and hardness distribution of the joints are analyzed,and thermo-mechanical analysis of different welding processes is performed. The impact of welding current on underwater welding temperature field and residual stress field is investigated. Under the same welding current conditions, the results show that the microstructures types and composition of underwater wet welded joints are different from those of onshore welded joints. For 170 A underwater welding,the microstructure of weld consists mainly of fine side lath ferrite and acicular ferrite,while the microstructure of coarse grained region consists mainly of martensite with high hardness. The amount of side lath ferrite in weld increases as underwater welding current improves.Simulation results show that lower welding temperature,faster postweld cooling rate and higher equivalent residual stress peak level can be obtained by using underwater welding with 170 A welding current comparing to onshore welding. As underwater welding current increases to 190 A,welding peak temperature increases while postweld cooling rate and residual stress level decrease.