The spray forming 7075 aluminum plates with thickness of 3 mm were welded by friction stir welding（FSW）using the same welding parameters under different media. The effects of different media on the joint of the thermal cycle curve,macro morphology,microstructure,hardness and mecha-nical properties were discussed. The results show that the thermal cycling curve of underwater welding tends to be more smooth and high temperature residence time is shorter. The weld surface of the underwater welding is forming better than that formed in the air. The grain of underwater joint is finer and the heat affected zone is narrow with clearly plastic manifold. The heat affected zone of under-air joint is softening seriously which hardness is only 113 HV. The fracture form of underwater joint is dissociation fracture,while the welded joint in air is fracture of quasi dissociation.