Abstract: A TIG-MIG hybrid welding system was set up.A series of experiments on high speed TIG-MIG hybrid welding of low-carbon steel were launched. The effect of TIG-MIG hybrid arc characteristic, droplet transfer mode and the weld pool behavior on weld bead appearance was researched and the welding parameters were optimized. Excellent arc stability and weld appearance were obtained by setting the same value for MIG and TIG currents in the range of 240-300 A. The penetration increased and weld width decreased compared with the traditional MIG welding. Humping or undercutting were suppressed at a high welding speed up to 2.5 m/min The suppression of humping and undercutting defects in TIG-MIG hybrid welding may be attributed to the decrease in hybrid arc pressure due to the interaction between TIG and MIG arc, the disappearance of gouging region and decreases in the weld width.