5000 series (Al-Mg) aluminum alloy for shipbuilding with thickness of 5mm was successfully welded by variable polarity plasma welding process at vertical-up position. The microstructure and mechanical properties of the joint were investigated. The ultimate tensile strength of the joint was 295MPa, the elongation was 10%, and the location of fracture was near the partly melted zone. The micro hardness of fusion line and heat affected zone was lower than the base metal, and the lowest hardness appeared in the center of the weld. The optical micrographs of the joint shown that the weld microstructure near the fusion line was coarse equiaxed grain structure and most of the weld region was cellular dendritic structure which grown from the two sides to the middle, while the microstructure of heat affected zone was typical recrystallization structure. The composition around the tensile fracture position was analyzed by EPMA, and the results revealed that the content of Mg was low and heterogeneously distributed. The lack and segregation of Mg and non-homogeneous microstructure decreased solution strengthening and resulted in a weak zone of the joint.