In this paper, heat-conduction underwater welding of Type-304 stainless steel with filler wire by using a 4kW Nd: YAG laser was investigated. In underwater welding, the shielding conditions of the local dry cavity have strong influence on the weld bead shape and weld properties. The structure of the nozzle and the gas flow speed are the main factors affecting the stability of the local dry cavity. In specific, appropriate enlarging the outside diameter of the nozzle can obtain perfect shielding condition. If the shielding condition is incomplete, the oxygen content of the weld metal may increase from the normal value of about 80ppm under good shielding condition to800ppm. The increase in oxygen content leads to the severe decrease in the elongation rate and the reduction of area in tensile test, although no variation in tensile strength. Under appropriate shielding condition, the chemical composition and mechanical properties o fthe underwater laser welds can be as same as that in air.