Abstract: The experiments of underwater wet friction taper plug welding were performed on API 5L X52 pipeline steel. The microstructure of joints with different welding parameters was observed by optical and scanning electron microscope, and the effects of welding parameters on microstructure were analyzed. The results show that the joints can be divided into forged zone (FZ), final frictional plane (FFP), shear deformation zone (SDZ), bonding zone (BZ) and heat-affected zone (HAZ). The SDZ presents layered structure containing coarse grains,and the microstructure of welds is extremely inhomogeneous. The axial force has a greater impact on microstructure of welds compared with rotational speed. The microstructure of underwater wet friction taper plug welded joint has a big difference with that of traditional friction welded joints which is characterized by fine-grained microstructure. These behaviors are caused by the welds mainly consisting of shear deformation zone during "closed mode" friction welding.