Abstract: Influence of welding speed and welding current on humping bead in the process of vertical high-speed gas metal arc welding (GMAW) was studied using the independently developed wall-climbing robot. The results show that when the welding speed or welding current exceeds a certain critical value, humping bead will generate in vertical high-speed GMAW. Moreover, the backflow liquid flow with high momentum generated by arc pressure, droplet impact force and gravity in the weld pool is the main reason for the formation of humping bead in vertical high-speed GMAW. Welding speed and current have a significant effect on the morphology of humping bead. When the welding current remains unchanged, as welding speed increases, both the spacing between humps and the height of hump decrease steadily firstly, then the rate of reduction decreases, while the width of bead decreases steadily. When the welding speed remains unchanged, as welding current increases, the spacing between humps increases firstly and then decreases, the height of hump increases firstly and then remains unchanged, while the width of bead increases steadily. In addition, the liquid metal appears to flow down when welding speed is too small or welding current is too large.