Abstract: The plasma underwater welding technology (PUW)was proposed and the welding process test was carried out on 304 stainless steel with a thickness of 5 mm. The optical microscopy, the super depth of field optical microscopy, tensile testing machine were applied to observe and to analyze the joint morphology as well as the mechanical properties. The results show that PUW can obtain stable arc and high-quality weld. The microstructure of weld metal mainly consisted of homogeneously distributed slightly coarsened dendritic crystal, some regions existed dense and uniform equiaxial dendritic crystal. The tensile strength of welded joints was about 77.56%. The fractured surface morphology presented typical ductile fracture. But the pores present on the weld surfaceaffect the mechanical properties of the weld.