Abstract: 5A05 aluminum alloy plate with the thickness of 1mm was welded with Nd:YAG laser beam. The effect of welding parameters including pulse energy, pulse duration, welding speed and defocusing distance on the evaporation loss of Mg element, depth of fusion, variation of Mg element content in weld metal and micro-hardness of weld-joint were studied and analyzed. The results showed that stirring in molten pool had a significant effect on the evaporation loss of Mg element. With the increase of stirring and the weld penetration, the evaporation loss of Mg element was decreased. Due to the effect of Mg element content and cooling rate, the maximum hardness of weld joint was approximately located at the fusion line. From the surface to the bottom of molten pool, the hardness of weld joint first decrease and then increase.