Abstract: The humping bead in vertical high-speed GMAW was studied by welding experimental platform with the independently developed wall-climbing robot in this paper. It was found that when the welding current remained unchanged and the welding speed increased to a certain critical value,humping bead would occur during vertical upward welding process. Through high-speed photography,it could be believed that the backflow liquid flow with high momentum generated by arc pressure,droplet impact force and gravity in the weld pool was the main reason for the formation of humping bead in vertical high-speed GMAW. When the vertical downward welding was carried out,the direction of welding and the tilt position of the welding gun were changed,so that the flow direction of the backward liquid flow generated by the arc pressure and the droplet impact force in the molten pool was opposite to the direction of gravity,which could effectively suppresses the formation of the humping bead. The result showed that when the welding current was 200 A and the welding speed was 2. 4 m/min,humping bead still would not appear. As a consequence,the welding efficiency was greatly improved.