In underwater laser beam welding, the shielding condition of the local dry cavity is one of the key factors affecting the weld quality. The variation in the weld bead formation and mechanical properties of the welded joint under various shielding conditions was investigated in this paper. The welding experiments were performed with Type3O4 stainless steel as base metal and Type-ULC308 electrode as the filler wire by using a 4kW Nd:YAG laser. The experimental results show that variation of gas flow speed has important influence on the weld bead formation. The bead width and reinforcement of the welds vary slightly under different gas flow speed, but the penetration depth is much lower under higher gas flow speed. Lower gas flow speed induces poor shielding conditions and increases the oxygen content of weld metal up to 800ppm, but higher gas flow speed that may form an excellent local dry cavity could prevent the weld metal being oxidized, with only about 8Oppm in oxygen content. Although the tensile strength of underwater laser welds keeps constant no matter how the shielding conditions are, the ductility of weld metal decreases largely for the weld obtained in poor shielding conditions comparing with that of good shielding conditions. The structure of the shielding nozzle has more important effect on the shielding condition, in specific, appropriately enlarging the outside diameter can considerably improve the stability of the local dry cavity.