Abstract: The fiber laser beam with high power density is used for welding 6A02 aluminum alloy with 1.0mm thickness. And the effects of heat input on weld macrography, structure and properties are studied. The results show that the stable fully penetration weld could be obtained when the heat input is controlled in the range of 8-12 J/mm and the welding speed is very high. The typical crosssection of the welds always presents near X shape. The characteristic of the welds could reduce the nonuniformity of welding temperature field. It also could reduce welding buckling and deformation. The columnar microstructure is formed near the fusion line. The mixed microstructures, including columnar grains and equiaxed grains, distribute in the center of the weld. The transition from columnar microstructure to mixed microstructures can be found from the fusion line to the weld center. With the heat input reduced, the microstructures of the weld zone are tendency to fine, the soften phenomenon near the fusion line is helpful to weak, the microhardness of the weld zone and the tensile strength of the joints are increasing slightly.