The influence of environmental humidity on the mechanical properties of GMAW joint was studied in a hyperbaric test chamber which was used to simulate underwater hyperbaric welding. The low carbon steel Q235C was used as base metal,and V-groove butt welding tests with GMAW were carried out under low,medium and high environmental humidities with other process parameters unchanged. Then,tensile,bending and charpy impact tests and analyses were performed to the joints. The results showed that the change of environmental humidity had little effect on bending performance of the joint,but tensile strength of the joint was increased and impact toughness of the joint was reduced with the increase of the environmental humidity.