The microstructure, the regular and mechanism of various parameters on formation of weld bead of Mg alloy AMCa403 using a laser welding were investigated. The results show that sound welds without major defects can be produced. Two welding modes of deep penetration welding, heat conduction welding were found, and heat input was found to be the main factor for welding mode and shape. The microstructure of weld metal is significantly finer than the base metal. At the same power, with the increase of welding speed, the microstructure of weld metal is much finer.