Abstract: The theory of stability of a liquid with free surface is applied to understand the ‘humping’ phenomenon in bead formation during high speed welding. A real time image sampling system is established to verify the theoretical analysis. The result shows that, when a free droplet transfer mode is applied in high speed welding, the workpiece is heated by the arc throughout the process and thus the molten pool can not solidify in time.As a result, the molten pool is very long, which gives way to the instability of bead formareron and pinch effect of the liquid metal,so that it finally leads to the humping phenomenon. Based on the analysis and experimental result, the short circuiting transfer mode is suggested to improve the bead formation in high speed welding. During short circuiting mode, the arc extinguishes periodically, so the heat transferred into the molten pool is decreased, this helps to avoid the molten pool to be extra long and in turn avoid the instability of the liquid metal.The results of experiments indicate that with short circuiting transfer mode, sound weld can be achieved in high speed welding.