Abstract: The main factors which affect the content of diffusible hydrogen and microstructure in the weld made by underwater shielded metal arc welding were analyzed. The results show that the water around the arc contributed little to the increase of diffusible hydrogen content which just increased about 0.41%. The main sources of diffusible hydrogen were organics or crystal water in the electrode covering and the waterproof layer, which increased the diffusion hydrogen content about 12.1% and 21.8%, respectively. In addition, the rust on the workpiece surface was not negligible for the increase of diffusible hydrogen content to about 28.2%. The microstructure in the welded metal consisted of bainite and a small amount of side plate ferrites, but the heat-affected zone contained a large amount of martensites.The highest hardness in the welded joint was over 4 GPa, and the hardness peak appeared near the fusion line. An under bead crack, typical cold crack, occurred in the heat-affected zone.