Abstract: In this investigation, through welding aluminum alloys 5052, 5083 by laser in different welding condition, the character of melting was compared and analyzed . The following two problems were studied: the evaporation of main alloying element Mg and the influence of the reaction force induced from metal evapor and thermal plasma on penetration. The experiment result confirmed that the increment of the shield gas flow rate in some extent and the change of entrance angle of laser beam would improve the meltability and increase the penetration depth, the suitable granularity of workpiece surface would improve the absorptivity of laser beam.