Abstract: To enhance the reliability of laser-arc hybrid welding, undercut formation and its remedy mechanisms during this process were discussed.The results demonstrated that laser can increase undercut critical speed of hybrid welding, which reaches 5 times than that of arc welding with appropriate welding parameters.Two undercut remedying mechanism resulted from laser-arc interaction were found during hybrid welding.The one is the surface tension state of three phases (solid, liquid and gas)at weld toe is changed by laser-arc synergic effects and form a resultant force pointing to the outside of molten pool.The other is the enhancement of flow speed and time of molten metal flowing from pool center to outer by the increasing of heat input and temperature gradient in molten pool.This faster flow drives molten metal to weld toe and avoid undercut, which is the main mechanism for restraining undercut.Furthermore, the experiential formula to undercut critical speed of hybrid welding and the optimal adjusting range of arc voltage were also obtained.