The microstructure, mechanical properties, hardness, rules and mechanism of various parameters on formation of weld bead of Mg alloy ACM522 using a fiber laser welding were investigated. The results showed that sound welds without major defects could be produced and the mechanical properties of welding joints were good. With the increase of laser power, the penetration depth and bead width of the weld bead all increased. The mechanism of welding transformed from the conduction welding mode to the deep penetration welding mode and the weld beads aspect ratio became higher with the increase of heat input. The microstructure of weld metal was significantly finer and the hardness of weld metal was higher than that of the base metal.