We have discovered a third welding Process-unstable-mode welding (UMW)under a certain condition, apart from the already known stable deep penetration welding (DPW) and stable heat conduction welding (HCW) in a high-power laser welding. UMW has a basic feature that the two welding modes (DPW and HCW) appear intermittently with extremely varying penetration depths and welding widths. In this peper, offects of welding parameters (focal cosition, laser power and moving speed) on laser welding mode and weld forming have been comprehensively studied. Double-U curves of laser welding mode transition have been obtained, which can be used to determine the process parameters of the three mentioned welding processes.