Abstract: In this paper,the porosity in the welds of local dry CO₂ gas shielded semi-auto-matic underwater welding was studied,and the effect of CO₂ gas flow rate on the formation of porosity was stressed.It was found that there were two kinds of porosity in the welds,the "screw" shape and the "earth worm" shape.Both were of the hydrogen type.The porosity decreased with the increase of CO₂ gas rate,with its shape changing from "screw" into "earth worm".A suitable rate of CO₂ gas flow should be set to form a stable cavity at the back side of the groove in the root run of sealing weld,an effective way to prevent porosity.